Can you block with a creature that entered the battlefield under your control this turn?

Summoning sickness (formally called the 'summoning sickness' rule under CR 302.6) prevents a creature from attacking or using activated abilities that include the tap symbol ({T}) or untap symbol ({Q}) in their cost. It does not prevent a creature from blocking.

CR 509.1 defines which creatures can be declared as blockers: the creature must be untapped and under your control. There is no requirement that the creature has been under your control since the beginning of your most recent turn — that restriction only applies to attackers (CR 302.6).

So even if a creature just entered the battlefield this turn — whether from your hand, a spell, a triggered ability, or any other effect — you can immediately use it to block during your opponent's combat phase (or even your own, in rare scenarios).

Concrete example: Your opponent attacks with a 3/3 on their turn. In response to nothing (before blockers are declared), you had cast a 2/4 Wall of Stone on your second main phase last turn — but even if you cast it this turn at instant speed via flash, you could still declare it as a blocker during the declare blockers step. It cannot attack next turn until it has been under your control since the start of your turn, but blocking is available immediately.

The key takeaway: summoning sickness restricts attacking and tapping via activated abilities, not blocking. A freshly played creature is a perfectly legal blocker.