The 22-year-old has yet to make a first-team appearance for the Latics, but he has featured for Brentford, Burton Albion and Accrington Stanley during previous loan spells.
"I'm delighted to have Rob signed," Southend manager Phil Brown told the club's official website. "We are a bit short defensively at the moment.
"He has impressed us all in training and comes here with experience."
Kiernan was included in Brown's starting XI for this afternoon's League Two clash at Burton.