A society matron forgot to put her $610,000 diamond ring back into the safe-deposit box at her Upper East Side bank — where a thief swiped it.

Miranda Tang, 80, who lives in the city’s most storied co-op, at 740 Park Ave., went to the Chase at 35 E. 72 St. last Tuesday to access her safe-deposit box, law-enforcement sources said.

Tang exited the bank but had forgotten to put her 7-carat emerald-cut ring away first.