Michelle Obama -- I'm Partying at Oprah's Maui Pad!

Michelle Obama is spending the last, fleeting days of her 40s at Oprah's palatial Maui estate ... law enforcement sources on the island tell TMZ.

The First Lady -- who turns 50 next Friday -- stayed behind when her hubby went back to the Oval Office. She island hopped from Oahu to Maui ... and went directly to Oprah's 12-bedroom spread.

A photog on the island tells TMZ ... Michelle will have company, as in Oprah.

Sources tell us security on the island is airtight, with SWAT teams and just about every other resource securing wide perimeters around Michelle.

It's truly a lady's vacation. Sasha and Malia went back with dad.