The left-back, 25, signed a 28-day emergency loan last month and had made five appearances, scoring once.
Millers boss Steve Evans was keen to extend the deal, but the Championship club were not willing to allow Harris to leave for a further month.
"Bob's 28-day loan has now expired," said Chief Operating Officer Paul Douglas. "The plan was to extend his stay with the Millers, but unfortunately Blackpool have requested that he returns to the club, and we've been unable to keep him any longer.
"Bob has been an excellent temporary acquisition and I'm sure the reports that Blackpool receive back from Steve will be very positive."
The blow has been offset by the recovery of injured left-back Laurie Wilson, who managed 90 minutes in the reserves following a knee problem.