The 28-year-old is set to remain in the Sunshine State for next year, with the Sharks revealing their newest recruit on Tuesday night.
The signing comes just days after Southport confirmed Lockhart would join the club for the 2022 VFL season, also adding to the likes of ex-AFL talent Aiden Fyfe, Jacob Heron, Zac Foot and Boyd Woodcock in linking up with the Sharks for next year.
Lockhart played 22 games for Melbourne, with his last appearance coming in Round 13 last year. The 25-year-old was delisted by the Demons just days after the club's drought-breaking premiership win.
Southport joined the new-look VFL competition in 2021 following the restructure of the second-tier competition, with fellow Queensland club Aspley also adding to the east-coast competition.
The Sharks managed nine wins from their 10 matches prior to the season being cut short due to the impacts of the COVID-19 pandemic, quickly emerging as a premiership threat.
Steve Daniel's side will continue to feature in the 21-team league next year in hope of a full campaign ahead, while Aspley have returned to the QAFL for 2022.