Cold brew tastes sweeter than hot black coffee even though it has almost no sugar. What usually explains that sweet impression?

Answer: Bitter and sour notes are quieter

More sugar dissolves in cold water — Coffee does contain small amounts of sweet-tasting compounds, so this guess has a foothold. But black coffee usually has far too little sugar for that to be the main taste signal. Cold brew often feels sweeter because fewer bitter and sour notes compete with the caramel-like and fruity aromas already present.

Bitter and sour notes are quieter ✓ — Sweetness here is mostly a change in balance, not added sugar. Cold brewing tends to pull out fewer sharp acids and bitter compounds, so the same aroma cues read as rounder and sweeter to your brain. That is why an unsweetened cold brew can taste smoother even when its actual sugar content stays tiny.

Caffeine tastes sweeter when chilled — Caffeine is part of coffee's bitterness, so making it more noticeable would usually push the drink away from sweetness. The caffeine level also depends on recipe and steeping time, not simply on whether the water was cold. The sweeter impression is better explained by weaker sour and bitter masking.