The US Customs and Border Protection agency said in a Monday statement that agents apprehended the group of 46 men and 24 females, including three minors, on Mona island.
Officials say the migrants are Haitians and were in decent health when they were found on Saturday.
The rugged island lies in a treacherous passage between Puerto Rico and the Dominican Republic. Smugglers in the Dominican Republic sometimes ditch migrants there.
Chief patrol agent Ramiro Cerrillo says "smugglers leave migrants stranded in Mona making them believe they safely have reached Puerto Rico."
A US Coast Guard cutter brought the migrants to Puerto Rico where they will be processed and transported back to Haiti.