An analysis said bitcoin could come under seasonal downside pressure in September.

CoinPost, a blockchain media outlet, reported on Aug. 28 local time that Japan-based research firm XWIN Japan pointed to a tendency for both U.S. stocks and bitcoin to weaken in September.

XWIN Japan said the S&P 500's average September return over the past 50 years is -0.8 percent, the weakest month of the year. Bitcoin also posted negative September returns for six straight years from 2017 to 2022.

The analysis said investors should look beyond bitcoin-specific factors and consider the broader flow in risk assets. Citing shared seasonal weakness in U.S. stocks and bitcoin, XWIN Japan said September this year calls for keeping the possibility of a correction in mind rather than relying on simple optimism.

It cited end-of-quarter portfolio adjustments, profit-taking after a summer rally and investors reassessing risk after the holiday season as factors behind September weakness in U.S. stocks. It also said policy uncertainty could increase volatility as the 2026 U.S. midterm elections overlap with the period.

XWIN Japan said it is difficult to draw firm conclusions about this year based solely on past statistics. It said there is no need to define September 2026 simply as a "selling period", and that the key is whether seasonal adjustments spread into broader risk-off sentiment. Past patterns can be a warning signal, but the actual price direction can be driven by other market variables.

Recent trends have also differed somewhat from the past. Bitcoin ended September higher for three consecutive years from 2023 to 2025. That means the September weakness pattern seen from 2017 to 2022 has recently eased. As a result, interest this year is expected to focus on whether seasonal downside pressure will strengthen again rather than on a simple repeat of past patterns.

Key variables include U.S. stocks and long-term Treasury yields, expectations for Federal Reserve rate cuts, and flows into spot bitcoin exchange-traded funds and spot demand. XWIN Japan said bitcoin is likely to face downside pressure if risk-off sentiment spreads across markets, but it also said seasonal weakness could be more limited if ETF inflows and spot demand remain firm.

Ultimately, in the September bitcoin market, capital flows and investor sentiment across risk assets are expected to be more important criteria than past statistics. Whether seasonal weakness in U.S. stocks turns into an actual correction, and whether ETF and spot buying offset it, remained key variables that will determine the short-term direction.