Baroness Goldie, who led the party between 2005 to 2011, said she would focus her efforts on the House of Lords following her elevation to the peerage two years ago.
David Cameron praised her as a “towering strength”, while current leader Ruth Davidson said she had been an “inspiration” and an “unstoppable force”.
She joins fellow Conservative MSPs Alex Fergusson, Nanette Milne, Gavin Brown and Mary Scanlon who are also standing down at next May’s Scottish Parliament election.
Baroness Goldie has been a West of Scotland MSP since 1999.
She played a leading role in the referendum campaign and its aftermath, when she sat on the cross-party Smith Commission on further devolution to Holyrood.
Baroness Goldie said: “It has afforded me both satisfaction and pleasure to serve my constituents and to serve the parliament and I will look back with great happiness at my time as an MSP.
“I am grateful to friends and colleagues from all parties for their support. Sometimes we found common ground, sometimes we disagreed but never, I hope, with rancour nor disrespect.
“Politics is a rough trade but we have built a strong parliament in Scotland of which we can all be rightly proud.”