On September 21, Glassnode’s Altcoin Cycle Signal entered “altcoin season”. Since August 19, the capitalization of altcoins has increased by 33%. At the same time, bitcoin dominance remains close to 60%. CoinMarketCap’s Altcoin Season Index remains well below its altseason threshold.
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Altcoins have finally joined the bitcoin rally
Glassnode’s Altcoin Cycle Signal left the “bitcoin season” regime on September 21 to enter the “altcoin season”.
Glassnode notes a clear difference with the month of August. The rally mainly benefited bitcoin, while altcoins remained flat. On September 21, altcoins participated more in the rise.
The cumulative capitalization of altcoins reached $1.19 trillion on September 22, its highest level since late January. Bitcoin had crossed $85,248 the day before, a level that had not been reached since January 29, 2026.
American spot bitcoin ETFs recorded $999 million in inflows on Monday, their best session since October 2025.
Glassnode signal can announce an altcoin season even when bitcoin falls
Glassnode compares the performance of a basket of 250 altcoins, excluding stablecoins, to that of bitcoin.
When this basket outperforms BTC, the indicator classifies the period as “altcoin season”. Altcoins can therefore be in altcoin season even when they fall, provided that they fall less than bitcoin.
On the graph published by Glassnode, the last 3 periods classified as “altcoin season” thus coincided with falls in bitcoin of 9%, 22% then 18%.
It is therefore an indicator of relative performance. Its score alone does not allow us to know whether altcoins are progressing strongly in absolute value. Glassnode also does not publish the exact formula used for its calculation.
The Altcoin Season Index remains far from the altseason threshold
On September 22, CoinMarketCap's Altcoin Season Index stood at 49, far from the threshold of 75 used by the platform to qualify an altseason. The index measures, over 90 days, the share of the top 100 cryptocurrencies that outperform bitcoin.
Bitcoin dominance reached 59.7% on September 22, up from 59.2% on August 19.
Over the same period, the capitalization of altcoins increased by 33%.
The 2024 altseason didn't last
At the start of December 2024, CoinMarketCap's Altcoin Season Index rose to 87. By March 2025, it had fallen below 25, into “bitcoin season” territory.
According to market maker Wintermute, altcoin rallies lasted around 19 days on average in 2025, compared to around 60 days in 2024.
Glassnode’s signal entered “altcoin season” on September 21, after the sharp rise in altcoins. With bitcoin dominance at 59.7% and CoinMarketCap's Altcoin Season Index at 49, the data remains mixed on the continuation of a sustainable altseason.
Yield can also come from other mechanisms
Altcoins are therefore regaining strength, but their recovery does not yet allow us to know how long it will last.
If your portfolio is entirely exposed to altcoins and you wait for an altseason to put your capital to work, you remain dependent on the next market movement.
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