That was up from the previous week's net purchases of $18.8 billion.
The purchases brought the U.S. central bank's purchase of mortgage bonds guaranteed by Fannie Mae, Freddie Mac and the Federal Home Loan Bank system to roughly $861.9 billion since January.
The Fed said it bought $31.22 billion gross of agency MBS from September 10 through September 16. At the same time, it sold $5.77 billion in mortgage securities.
The Fed aims to buy up to $1.25 trillion of agency MBS in a bid to bring down mortgage rates and to stimulate the battered housing sector and the overall economy.