BIST Index Rules Revamped: Major Changes in Market Structure

Extensive Updates to Stock Indices Rule Set from Borsa Istanbul

Borsa Istanbul announced that it has made significant updates to the share indices rule sets, which are periodically rigorously reviewed by the Financial Criteria Committee. Shaped with the aim of adapting to dynamic market conditions and international standards, these changes aim to increase the depth, transparency and reliability of Turkish capital markets. These strategic regulations, which take into account valuable insights from market participants, are final, in order to provide market actors with an adaptation period August 27, 2025 will officially enter into force from the date.

These regulations, beyond just being a technical adaptation, are aimed at strengthening the representativeness of Borsa Istanbul indices and providing investors with clearer and more reliable indicators. These steps, taken in line with evolving financial technologies, changing investor expectations and trends in global markets, will contribute to Turkey's vision of becoming a financial center. The fact that indices most accurately reflect the overall health and direction of the market plays a critical role in the decision-making processes of both local and foreign investors. In this regard, Borsa Istanbul's proactive approach is a concrete indication of its efforts to increase the effectiveness of market mechanisms and strengthen investor confidence.

Fundamental Points of Comprehensive Changes and Rationality

The updates foresee structural improvements in a number of critical areas. These changes are aimed at meeting the expectations of market participants and ensuring the long-term stability of the indices, while improving the quality of the indices. Each item is the product of a thought process that will put the market structure on more solid foundations.

Pre-Market Trading Platform (PÖİP) Shares Are Excluded From Index Coverage

Within the scope of the regulations made, Pre-Market Trading Platform (PÖİP)Shares traded in BIST All and BIST 500 Indices It was decided to exclude it from the scope. PÖİP is a platform on which shares of companies with lower market capitalization and liquidity are traded, which are usually subject to initial public offerings or that for various reasons have not yet been included in the main markets. Shares on this platform can naturally carry a higher risk profile compared to companies in the main market, have a narrower investor base and can be characterized by lower trading volumes.

The main rationale for this decision is to enable broad-based indices such as BIST All and BIST 500 to more accurately reflect the overall performance and liquidity structure of the main markets. The removal of PÖİP shares from the indices will improve the quality of the indices, allowing it to focus on more liquid and transparent assets. This will allow investors to assess market risks more soundly, while offering a more stable foundation for index-based investment products and funds. Thus, the market segment represented by the indices will become more homogeneous, more mature and more in tune with mainstream market dynamics. This step is also a parallel approach to the general trend of international index providers.

BIST 500 Index Selection Based on Market Structure

BIST 500 IndexIn the selection of shares to be included in, Borsa Istanbul Stock Market Market in accordance with the structure Yıldız Market, Main Market and Sub Market It is indicated that an evaluation will be made with the ranking. This new approach aims to prioritize the quality and liquidity level of the market segment in which the companies are located when selecting the components of the index. Star Market companies have the highest market capitalization, transaction volume and corporate governance standards, while Main Market and Sub-Market companies are also ranked according to established criteria. This hierarchy reflects the depth of the markets and the operational size of the companies.

This arrangement will enable the BIST 500 Index to more effectively represent the strongest and most liquid companies in the Turkish capital markets. Based on the market structure in the selection of shares to be included in the index will improve the overall quality of the index and its credibility with the investor. The gaining weight of more transparent and liquid companies with high standards of corporate governance in the index could increase the attractiveness of Turkish markets, especially for international portfolio managers and funds. This methodological improvement is based on the principle of better reflecting the market depth and institutionality of indices and aims to provide a more reliable reference point for those who implement passive investment strategies.

Index Evaluation Data Periods Extended from 6 Months to 12 Months

Used in the periodic valuations of a large number of important indices “Daily Average Trading Volume” and “Adjusted Price” the duration of their data, From 6 months to 12 months removed. This change, BIST 30, BIST 50, BIST 100, BIST 500, BIST Liquid Bank, BIST Non-Bank Liquid 10, BIST Sustainability 25, BIST Dividend 25, BIST Participation 30, BIST Participation 50 and BIST Attendance 100 It will apply to indices. Doubling the data duration will allow indices to reflect more robust and long-term market dynamics, while creating a methodology that is consistent with other appraisal data.

The main objective of this step is to reduce the impact of short-term market fluctuations or seasonal effects on index compositions. The 6-month data period could sometimes cause momentary or temporary market movements to be reflected in an exaggerated way in index valuations. The data assessment, spread over a 12-month period, more comprehensive and denser the performance and liquidity characteristic of companies It will make it possible to analyze it in a clear way. In this way, company changes in indices will take place on a more stable basis, providing a more predictable environment for fund managers and investors of index-based products. Many international exchanges similarly prefer longer data evaluation times, which also reinforces Borsa Istanbul's alignment with global best practices.

The longer data period encourages consideration of more persistent liquidity and price movements, rather than just instantaneous market popularity in determining companies' place within the index. This can contribute to the fact that indices experience less “churn” (frequent company changes) and lower transaction costs for investors. Furthermore, the need for market participants to assess companies' positions in indices based on their longer-term performance will give them a deeper and strategic perspective on decision-making processes in the market. This approach will strengthen the character of indices as true representatives of the market.

Ranking Ranges Expanded for BIST 100 and BIST Participation 100 Indices

BIST 100 and BIST Attendance 100 lower and upper ranks used in the selection of shares ranked in indices, 90-110 from 95-105 updated as. This regulation is an important step towards strengthening the “buffer zone” mechanism of indices. In order for a company to enter or not exit the index, it must be in a certain ranking. This range aims to prevent companies from entering and exiting the index frequently due to small market fluctuations or momentary performance changes, thus improving the stability of the indices.

The expansion of the ranking range (90-110) will allow the indices to gain more stability. For example, while a company that was previously ranked 106th faces the risk of being delisted from the index, with the new regulation, companies up to 110th place will be able to maintain the potential to remain in the index. Similarly, it has become possible that a company that is just entering the index can be included even with a lower ranking (up to 90th place). This can reduce the frequency of changes in indices, reducing the management costs of index-based products and offering a more predictable index structure for investors. Thus, in accordance with the fundamental purpose of the indices, it is ensured that they reflect the general trend and performance of the market in a more stable way. This flexibility prevents index components from changing unnecessarily during periods of extreme volatility, providing a healthier reference to market participants.

Update on “BIST Buyback Index”

Finally, Capital Market Board (SPK)due to the expiry of the relevant Principle Decree of “3.29 BIST Buyback Index” The section titled will also be updated. This technical regulation is a step towards ensuring the full compliance of Borsa Istanbul with market legislation and the regulatory framework. Companies” own share buyback programs are an important mechanism that affects market liquidity and share prices. The change in the principle decision of the SPK in this regard has forced Borsa Istanbul to revise the relevant index methodology as well.

This update demonstrates BIST's commitment to keeping index rules up to date at all times and adapting quickly to regulatory changes. Changes in the methodology of the repurchase index, in line with the new regulations of the SPK, are aimed at more accurately reflecting the effects of share buybacks on index performance and to provide market participants with up-to-date information on this issue. Thanks to this, the indices will be able to fully reflect all legal and operational dynamics in the market. This technical revision is critical to maintaining alignment between the policies of market regulators and the functioning of Borsa Istanbul.

What does it mean for investors and market participants?

These comprehensive index ruleset updates implemented by Borsa Istanbul's Financial Criteria Committee have been shaped in line with dynamic developments in the markets, changes in international legislation and implementation needs. These decisions will contribute significantly to bringing Turkish capital markets closer to international standards, increasing transparency and strengthening investor confidence. For Turkey, which aims at integration with global financial markets, such steps will positively affect market perception.

The new rules August 27, 2025 The date of entry into force gives market participants sufficient and reasonable preparation time to review their portfolio strategies and investment models in accordance with this new structure. Achieving a more stable, liquid and representative structure of indices is especially important for passive fund managers, institutions offering index-based investment products and algorithmic trading investors. With these regulations, Borsa Istanbul continues its vision to offer a more mature and predictable market environment. These steps, which lay a solid foundation for the future growth and development of Turkish capital markets, should be carefully followed by market actors and strategies should be optimized accordingly. Borsa Istanbul demonstrates its determination not only to improve the current situation with these steps, but also to build a structure more resistant to future market changes.