Peterborough United's rivals appoint a head coach recently sacked by a League Two club
Harris, who was recently sacked by League Two side Gillingham, replaces Mark Bonner who was dismissed by Cambridge just three weeks after a 5-0 thrashing at Posh.
Harris has previously managed Millwall and Cardiff City. He steered Millwall to promotion from League One via the play-offs in the 2016-17 season.
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Hide AdCambridge have been in freefall in League One, losing their last three games to drop to 18th place, but they did beat Fleetwood 4-0 in the FA Cup last weekend.
Harris will make his debut in the dugout in a League One game at Charlton Athletic on Saturday.