The Dolphin Slash (ドルフィンスラッシュ Dorufinsurasshu) is Marth's up special move. Marth flies quickly upwards, with his blade extended vertically. It provides decent vertical recovery, but very little horizontal recovery. It can be used as an out of shield option and also to edgeguard an opponent.
Dolphin Slash's damage and knockback is strongest in its first few frames of attack. The longer the attack is out, the weaker it gets. It's best used point blank if it's meant to be used as an attack, since the later frames of the attack are weaker than the beginning frames. It can be slightly angled forward, similarly to Roy's Blazer.
 Reverse Dolphin Slash
A "Reverse Dolphin Slash" can be performed by starting the attack and quickly moving the joystick backwards. This will cause Marth to turn around just before jumping upwards, and if it hits an enemy close to him as he starts the move, it provides much higher damage and knockback due to the hitbox and sweetspot layout of his sword. Due to these properties, the Reverse Dolphin Slash can act as both a powerful surprise KO move as well an effective move to gimp offstage. Often Marth players will use this move as an alternative finish to a Ken Combo should they be too close to sweetspot Marth's down air spike. Also due to it's speed, some players will use the move to punish a poor tech roll.
In Brawl, the regular Dolphin Slash appears to be at least as powerful as Melee's Reverse Dolphin Slash. Marth can be seen jump-cancelling a shield into it for potential kills at about 100% (130% on heavyweight characters).
This move in Brawl has several more advantages than the version in Melee. It goes slightly more horizontal, can be angled more noticeably forward, and its vertical recovery is still great. It is also one of the few moves to retain the similar speed in Melee. However, it lacks the higher potential to KO in comparison.
What makes this move really useful are the invincibility frames starting on frame 1 to 5, allowing Marth to escape chain grabs and multi hit attacks. However, when used for this purpose, Marth is left helpless and vulnerable to attack. The move hits the opponent on frame 5, like it was in Melee.
 In Super Smash Bros. 4
Dolphin Slash is confirmed to return in Smash 4.