The versatile 18-year-old, who can play in attack, on the wing and has featured at left-back, has signed for an initial month in time to be included in the Shrimpers' squad for Saturday's League Two trip to Rotherham.
Southend manager Paul Sturrock told the club's official website: "We're pleased to have Jonson on board for the next four weeks.
"He's an exciting talent and we've watched him twice, so we'll see how he gets on in the next month."
Clarke-Harris began his career at Coventry, becoming their youngest-ever debutant in a Carling Cup defeat at Morecambe two years ago, and following his release from the Ricoh Arena in the summer signed a two-year contract at Peterborough.
