Which Mega Man game introduced the ability to slide?

The Mega Man game that introduced the ability to slide was Mega Man 3.

Released for the Nintendo Entertainment System in 1990 in Japan and in 1990–1991 in other regions, Mega Man 3 added sliding as a new movement technique. Pressing down and jump lets Mega Man skim along the ground, temporarily reducing his height and moving quickly beneath hazards.

The move changed how players approached both exploration and combat. Mega Man could slip under low attacks, pass through narrow one-block spaces, and avoid obstacles that could not be cleared with a normal jump. It also became useful for maintaining momentum through stages and escaping enemies at close range.

Mega Man 3 introduced several other enduring elements, including Rush, Mega Man’s robotic dog, and Proto Man. Sliding became a standard part of Mega Man’s classic moveset, although it was deliberately absent from Mega Man 9 and Mega Man 10, which returned to the more limited movement style of the earliest games. The ability therefore belongs specifically to the classic series’ third numbered installment.