Can you block with a creature that has summoning sickness?

A creature with "summoning sickness" — meaning it has been under your control since the beginning of your most recent turn — is restricted by CR 302.6, which states that a creature cannot attack or activate abilities with the tap (or untap) symbol unless it has haste. Blocking is not on that list.

Blocking is declared during the defending player's opportunity in the Combat Phase (CR 509.1). The rules require only that a blocker be an untapped creature you control; there is no requirement that it has been under your control since your last upkeep. Summoning sickness simply does not apply to blocking.

Haste (CR 702.10) removes the summoning-sickness restrictions on attacking and using tap/untap abilities, but a creature doesn't need haste to block — any untapped creature you control may block regardless of how recently you gained control of it.

Concrete example: Your opponent attacks you on their turn. You just cast a 2/2 Bear token on your turn, so it has summoning sickness. You can still declare that Bear as a blocker to stop the attacker — summoning sickness poses no obstacle to blocking.

Keep in mind other abilities can still prevent blocking (e.g., Defender stops blocking too, and effects that say "this creature can't block" override the general rule), but summoning sickness alone is never one of those restrictions.