The 40 year old politician, who leads for his party during Scottish government budget debates, has served as a Lothians list MSP since 2007.
Mr Brown said he will quit his role as Scottish Conservative finance spokesman with immediate effect due to his decision to leave Holyrood in 2016.
Conservative MSP Murdo Fraser will take over Mr Brown’s frontbench role.
Mr Brown said: “I have decided not to seek re-election to the Scottish Parliament in the 2016 election and will stand down at that point.
“I will have served nine years by 2016 and would like to seek a fresh challenge going forward.
“Given that I will not seek re-election, I am standing down from my role as Scottish Conservative finance spokesman.
“This decision takes immediate effect. It has been a thoroughly fascinating role at a key time in Scottish politics.”