Progress in the nasty divorce involving Kim Zolciak and Kroy Biermann ... it appears they've hammered out a child custody agreement ... but it's only temporary.

The estranged couple sat down for a mediation session Tuesday last week, according to new legal docs obtained by TMZ ... which resulted in a temporary custody agreement signed by Kim and Kroy.

It's unclear what the custody agreement entails ... all the docs say is they signed a deal.

Kim and Kroy have four minor children ... sons KJ, Kash and Kane, plus daughter Kaia.

The ugly divorce has been dragging on for over 2 years now ... but this appears to indicate some signs of progress. At one point last year, Kroy had even pushed for primary custody of all four kids.

We reached out to Kim's reps and Kroy's camp ... so far no word back.