The 21-year-old, who has been on the bench for a number of Barclays Premier League games this season, joins the Posh on a temporary basis until 3rd May.
Isgrove has played for Saints twice in the Capital One Cup, and now heads to London Road in search of his first taste of league football.
Peterborough currently sit sixth in the League 1 standings with 11 games to go, and face Chesterfield in the final of the Johnstone's Paint Trophy at Wembley later this month.
Executive Director Les Reed said: “As it is generally not our policy to put large numbers of our young development players out on loan, we can be selective when assessing opportunities for the more senior players to gain unusual or beneficial experience.
“Lloyd has a fantastic opportunity to experience playing in a team competing for promotion and for victory in the Johnstone's Paint Trophy.
“He has had significant experience training with our First Team and being in the squad and on the bench, so is now ready for this short-term challenge.
“Peterborough play the correct way and with a style which will suit ‘Izzy’. We look forward to monitoring his progress of the rest of the season and welcoming him back in the summer for the challenge of the new season.”