Mellis, released from Chelsea in March after setting off a smoke grenade at the club's training ground, is poised to sign a two-year deal with the Tykes later this week, according to BBC Radio Sheffield.
The 21-year-old made 15 appearances for Barnsley during a loan spell at Oakwell under previous manager Mark Robins in 2011.
Mellis, who joined Chelsea from Sheffield United in 2007, made one appearance for the Blues in the Champions League in 2010.
Mido, meanwhile, is set for talks with Barnsley on Thursday.
The well-travelled former Tottenham, Middlesbrough, Wigan, Roma, Marseille and Ajax striker is a free agent after leaving boyhood club Zamalek following a pay dispute.
The 29-year-old target man has struggled with fitness problems since a fine debut season for Spurs in 2005 and last played for Egypt in 2009.
He scored three goals in four matches for Zamalek last season but refused to train after his wages were withheld, eventually leaving the club after falling out with officials.