The City of London officer was hurt when he confronted four moped riders as they tried to steal another moped from a man on Maylons Road, Lewisham, at about 17:45 BST on Saturday.
The unarmed officer, 37, was stabbed in the leg with a kitchen knife when he challenged the suspects.
He continued to challenge them until they fled the scene.
The officer was taken to a south London hospital with non life-threatening injuries, the Metropolitan Police said.
The 21-year-old owner of the moped was also assaulted.
The Met is appealing for witnesses and information.