It is believed the Blues’ League One rivals are one of a number of clubs interested in the 25-year-old free agent.
Johnson will officially be available for a free transfer this week after he was told he had no future at Fratton Park.
New Addicks boss Ben Garner has already stamped his authority at The Valley, recruiting four new faces to date this summer.
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And if reports are true, the former Accrington ace would join Mandela Egbo, Eoghan O’Connell and Joe Wollacott in moving to the south London outfit so far this window.
A Pompey reunion of sorts would also be on the cards for Johnson, with former Fratton favourite Craig MacGillivray having made the move to Charlton last summer.
Interestingly, John Marquis has also been linked with a move to The Valley – after he left Lincoln at the end of the season.
Following Johnson’s £100,000 move to Fratton Park from Accrington in 2020, the right-back accumulated 44 appearances for Pompey – scoring once.
He polled third in The News/Sports Mail Pompey Player of the Season award after that 2020-21 season.
Despite a successful maiden campaign at PO4, though, his Blues career was prematurely cut short as he fell down the pecking order under Danny Cowley.
The Pompey boss viewed Kieron Freeman and Mahlon Romeo as his preferred options at right-back, despite choosing Johnson in 11 of his first 12 games during his spell as interim coach.
The ex-Middlesbrough man would go on to spend the 2021-22 campaign on loan at Fleetwood, where he impressed in 39 outings for the Cod Army.
Johnson’s last appearance for the Blues came in a 2-1 defeat to Millwall last August, where he was shown his marching orders in the 48th minute, bringing an end to his Fratton Park career.