Can you block with a creature that has summoning sickness if it was under your control last turn?

Summoning sickness (the colloquial term for the rule in CR 302.6) prevents a creature from attacking or using activated abilities with the tap symbol (or untap symbol) in their cost — but it does not prevent a creature from blocking.

CR 302.6 states that a creature cannot attack or use such activated abilities unless it has been under its controller's continuous control since the beginning of that player's most recent turn. Blocking is governed by a completely separate set of rules (CR 509) and has no such restriction.

So if you played a creature on your turn, your opponent attacks you on their turn, and you want to block with that freshly played creature — you are fully allowed to do so, even though it cannot attack yet on your next turn unless it has been under your control since the start of that turn.

Example: You cast Grizzly Bears on your main phase. Your opponent attacks you with a 2/2. You can declare Grizzly Bears as a blocker during your opponent's combat phase, even though Grizzly Bears has summoning sickness and could not attack if the roles were reversed.

The key takeaway: summoning sickness restricts attacking and tapping for abilities, not blocking. Always check CR 302.6 for the exact scope of the restriction.