St Mirren informed the Scottish Professional Football League they only had 12 registered players who were fit, four of whom were goalkeepers.
"Due to a number of positive tests for COVID-19 amongst their playing squad and coaching staff, they have only 12 registered fit players," the SPFL said in a statement.
"As a result, St Mirren are unable to fulfil today's fixture."
It is the second time Motherwell have had a match postponed this month after Kilmarnock's squad was forced to self-isolate when they had six players test positive for the coronavirus. (Reporting by Rohith Nair in Bengaluru, editing by Ed Osmond)