The POW Block is a hazard in Super Smash Bros.'s Mushroom Kingdom stage. One POW Block appears at intervals in certain locations. Attacking it shakes the screen and deals 20% damage and considerable upwards knockback to every character currently touching the ground or a platform, often KO'ing characters over 100%.
The POW Block returns in Super Smash Bros. Brawl in one of the stages based one of the games it actually appeared in (they didn't appear in Super Mario Bros.), namely the stage Mario Bros. This time around, the block deals no damage, but it causes grounded characters to bounce upwards (which interrupts charging smash attacks and the like). It also lands a hit on every grounded enemy on the stage, and disappears in three hits (returning after a while).
The POW Block also returns as an item in Super Smash Bros. (Wii U and 3DS). Whenever it is thrown or hit, it causes an earthquake to harm all grounded opponents and cause them bounce upwards, dealing 10% damage (the earthquake also causes the POW Block user to bounce slightly, but it causes no damage to him/her). A single POW Block can be used 3 times.