If a creature gains haste the same turn it enters the battlefield can it attack that turn?

Normally, a creature cannot attack or use tap abilities (indicated by a tap symbol in the cost) on the same turn it enters the battlefield under your control. This restriction is informally called 'summoning sickness' and is grounded in CR 302.6, which states a creature cannot attack unless it has been under its controller's control since the start of their most recent turn.

Haste (CR 702.10) is a keyword ability that specifically overrides this restriction. CR 302.6 explicitly notes that creatures with haste are exempt — they can attack and use tap abilities the very turn they enter the battlefield.

It does not matter whether the creature had haste when it entered or gained it later in the same turn. As long as the creature has haste at the time you declare it as an attacker (during the Declare Attackers step, CR 508.1), it is free to attack.

Concrete example: You play Goblin Guide (which has haste printed on it) on your turn. When your combat phase begins, you can immediately declare Goblin Guide as an attacker, because haste lets it ignore the summoning sickness rule. If instead you played a vanilla 2/2 and then a spell gave it haste later that same turn, it could also attack.

One important caveat: if a creature loses haste before the Declare Attackers step, it would be subject to summoning sickness again and could not attack that turn.