Don Lemon is on thin ice at CNN — although he’s being welcomed back for now, TMZ has learned he’s a strike away from getting the boot permanently, and he knows it.

Sources with direct knowledge about Don’s situation tell us … when the veteran anchor spoke with upper management this week to discuss his sexist comments and the path forward, he was told this was his final warning on such antics.

We’re told it was communicated to Don in no uncertain terms … the next time he insults his cohosts or creates a toxic environment (on or off-air) will be his last with the network.

Of course, we know Don agreed to these terms in addition to signing off on participating in formal training of some sort. The guy’s set to return to his job Wednesday, so he’s up to speed on all this.

It’s interesting Don was given this ultimatum, especially since we’ve heard from people in his world that he’s not happy at CNN — not at all — so the uneasiness is mutual.

As for why Don was given such a short leash — well, the fact is … he’s pretty polarizing. DL’s racked up negative headlines (for one reason or another) in recent years, and not just from criticism from people on the right.

There’s another factor here too that must be on the minds of Chris Licht and co. … ratings. Not only were Don’s numbers for his former primetime slot not strong enough to warrant him sticking around there … ratings on his new gig have tanked.

Plus, we’re hearing staffers at CNN are still on edge over Thursday’s incident … with one source saying the anger is real and hasn’t died down. It’s truly become a distraction.