Bitcoin climbs even as an altcoin season remains on the table. In September 2025, the Altcoin Season Index hit 78 while Bitcoin’s share of total crypto value slipped from 62% to 55%. These readings track how money moves and shape decisions for traders, treasuries, and institutions.
A rising Bitcoin and stronger altcoins are not mutually exclusive — both can advance if certain conditions line up. A falling Bitcoin dominance, a rising ETH/BTC price ratio, a higher combined altcoin market value, and climbing spot and derivative volumes.
The Altcoin Season Index counts how many of the top 50 altcoins beat Bitcoin over a 90-day window; a reading above 75 flags broad outperformance.
Reading the pulse of an altseason
The Altcoin Season Index tracks how many of the top 50 digital assets outperform Bitcoin within a 90-day period. When the reading surpasses 75, it signals broad outperformance across the sector. In September 2025, the index climbed to 78, placing the market in what many interpret as the early stages of an altseason. This metric has become a valuable tool for gauging rotation away from Bitcoin and into the wider altcoin market.
Technically, the Altcoin Season Index is a composite measure that records the share of altcoins posting stronger returns than Bitcoin over a set timeframe. Analysts often emphasize Ethereum’s critical role in confirming such phases. For some, a new all-time high in ETH is the true trigger that validates a lasting shift in market dynamics, reinforcing Ethereum’s status as the benchmark for altcoin strength.
If current signals hold, capital may migrate more quickly from Bitcoin into smaller tokens. This flow reshapes both liquidity conditions and risk profiles. For institutional treasuries and funds that operate under strict liquidity and governance frameworks, the drop in Bitcoin’s dominance from 62% to 55% creates space to diversify selectively into alternative assets, particularly as the ETH/BTC ratio trends upward.
Meanwhile, derivatives markets play a pivotal role in this rotation. Traders closely watch funding rates and perpetual swap basis levels to exploit arbitrage opportunities. Yet, increased altcoin volatility brings higher liquidation risks, meaning leverage-heavy strategies must balance opportunity with the possibility of rapid losses.
Finally, historical cycles remain an important guide. Altseasons have often emerged six to nine months after a Bitcoin halving, suggesting that timing continues to matter as much as technical signals. With aggregate altcoin market capitalization and trading volume on the rise, the backdrop is set for potentially heightened swings, and investors must weigh the allure of outperformance against the danger of overheating.
