Zac Brown wants a judge to force his estranged wife to take down an Instagram post that he says is incredibly damaging to him ... and he's run to court to make it happen STAT.

The country singer filed a lawsuit Friday against Kelly Yazdi -- whom he married in August ... only to split up with her four months later through a joint statement, amounting to a complete 180 on their relationship.

According to court docs, obtained by TMZ, Zac claims that an Instagram post Kelly has up on her feed is in violation of confidentiality agreements she has signed with him.

It's unclear which post exactly he's talking about -- but ZB says in his docs that the IG upload violates her confidentiality obligations ... and he wants the judge to block her from making false, defamatory or otherwise harmful statements.

Because he feels she's in violation of this ... Zac is seeking emergency injunctive relief in the form of a temporary restraining order that'll make her take down the IG post swiftly. At this point, it doesn't appear the court has signed off on the request.

Remember ... Zac married this woman in August last year, and in December ... they both came out and said they were dunzo and on their way toward divorce.

What's interesting about that ... as far as we can see, there's no public record of a divorce case for these two just yet ... but despite that, Zac's lawyers note he has since filed for divorce after splitting up with Kelly. Either way ... they're definitely not on good terms.

We've reached out to Zac and Kelly for comment ... so far, no word back.