Venus has the hottest average surface temperature of any planet in the Solar System. Its surface averages about 465°C, even though Mercury is closer to the Sun.
The reason is Venus's extraordinarily dense atmosphere, which is composed mostly of carbon dioxide. This gas traps outgoing heat in a powerful greenhouse effect. Thick clouds of sulfuric acid surround the planet, preventing much of the heat from escaping into space.
Venus rotates very slowly and in the opposite direction from most planets. A Venusian day, measured from one sunrise to the next, lasts about 117 Earth days, while a full rotation relative to the stars takes about 243 Earth days. Its year is shorter, lasting about 225 Earth days.
Mercury experiences more extreme temperature swings because it has only a very thin atmosphere. Its dayside can become extremely hot, but its nightside cools dramatically. Venus's atmospheric blanket keeps temperatures high across the planet, making it hotter on average than every other planet.