Stock Market: Will S&P 500 Open Up or Down Today?

Sep 21, 2026
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U.S. stock futures are trending higher early Monday as investors digest a stark warning from President Donald Trump to Iran alongside cooling crude oil prices, while looking ahead to a week of key consumer retail earnings.

The Polymarket (CRYPTO: POL) crowd is heavily bullish for the Sept. 21 trading session. The “S&P 500 (SPX) Up or Down on September 21?” contract currently reflects an 81% chance of a higher open.

Sept 21 Polymarket Odds For S&P 500 Opening

Why That Number Matters

Traders are navigating a mix of geopolitical escalations, AI sector debates, and a busy week of economic insights:

  • Positive Index Futures: Equity futures point to a green open across major benchmarks. Dow Jones futures rose 0.35%. S&P 500 futures advanced 0.41%. Nasdaq 100 futures gained 0.56%, and Russell 2000 futures ticked up 0.34%.

  • Geopolitics & Energy: President Trump warned Iran of severe economic consequences or the destruction of its leadership if Tehran refuses to reach a deal. Iran’s military responded by threatening sustained retaliation against U.S. bases. The confrontation widened over the weekend with Houthi missile and drone strikes on sensitive targets in Riyadh. Despite this, oil prices retreated; Brent crude dropped to $102.05 a barrel, down 1.75%, and WTI crude fell to $94.33 by 1.82%.

  • AI Developments: Over the weekend, President Trump polled the public on renaming artificial intelligence, suggesting terms like “Superior Intelligence,” and proposed a new “AI Force” and an “AI Czar” to monitor the industry. This comes as the U.S. and China prepare for bilateral discussions covering AI safety, with China recently accusing the U.S. of applying “naked double standards” on AI model distillation.

The Bull Case and Market Outlook

Despite the recent wave of central bank rate hikes, the global stock bull market remains remarkably resilient, according to Ed Yardeni. “Central banks in the US, Europe, and Japan all raised their policy rates over the last two weeks,” yet “neither development has broken the global stock bull market, as earnings forecasts keep getting marked up,” he said.

How the Previous Bet Played Out

The Sept. 18 Polymarket contract resolved “Up,” recording $94,541 in total trading volume.

On Friday, the SPDR S&P 500 ETF Trust (NYSE:SPY) and Invesco QQQ Trust ETF (NASDAQ:QQQ), which track the S&P 500 and Nasdaq-100, respectively, closed higher. SPY rose 0.13% to $761.69, while QQQ rose 0.63% to $721.45. Meanwhile, the Dow tracker, State Street SPDR Dow Jones Industrial Average ETF Trust (NYSE:DIA), ended 0.25% lower at $515.88.

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