Swallows, who join AmaZulu in the NFD next season, were undone by Roggert Nyundu's 31st minute strike at Thohoyandou Stadium.
The win lifted Leopards three points clear of Jomo Cosmos in the Play-Off standings, with both sides level on goal difference.
Ezenkosi will play the last game of the Play-Offs against the Dube Birds in Soweto on Wednesday.
Usuthu were demoted on the final day of the 2014/15 PSL season, with their place in the topflight being taken by NFD champions Golden Arrows.
The match started off at a high tempo, with Nyundu firing a set-piece straight to goalkeeper Greg Etafia in the 15th minute.
Nyundu then managed to beat Etafia with a rocket of a shot which gave the Nigerian no chance.
A few minutes later, Craig Rosslee’s charges were under pressure when they conceded a free-kick. Sphelele Ntshangase delivered a great curler which Etafia parried over for a corner.
As both teams went to the dressing room with the hosts leading by 1-0, the Dube Birds came back stronger with substitute Luyolo Nomandela seeing his effort saved by keeper Avhashoni Tshinuna a minute after the restart.
Nyundu, though, nearly made it 2-0 when he was denied by Etafia in the 66th minute from a set-piece as he was looking for the top corner.
Two minutes later, a good run from Luyolo Nomandela saw the Mamelodi Sundowns man combining well with Siyabonga Nomvete who laid it back for the oncoming Lerato Chabangu, however ‘All You Need Is Love’ couldn’t keep his effort low as it flew over Tshinuna’s posts.
In the last 10 minutes, both camps ushered instructions into the field but it was the home side who was comfortable and defended well as they walked away with the three points.
Leopards: Tshinuna, Nyundu, Nyirenda, Maphangule, Ntshangase, Mabotja, Mumuni, Madimba (Shivambu 57’), Ndou, Munyai, Mahangwahaya (Munganga 90+2’).
Swallows: Etafia, Mngomezulu, Hendricks (Chabangu 54’), Isaacs, Lekgothoane, Baloyi, Sibande, Mandic (Phalane 45’), Wana, Nomvete, Yende (Nomandela 45’).
PSL Promotion/Relegation Play-Off log:
P W D L F A Diff Pts
Leopards 4 2 1 1 2 2 1 7
Cosmos 3 1 1 1 4 3 1 4
Swallows 3 1 0 2 3 4 -2 3