"It has been an honour to be the Member for Dunstan, leader of the South Australian Liberal Party and premier of South Australia," Mr Marshall said in a statement on Wednesday.
"After almost 14 years in parliament, now is the right time for me to step aside and allow someone else to step forward and serve the people of Dunstan.
"I will leave politics knowing I did my best and fought my hardest for this great state."
Mr Marshall, 57, served as 46th premier of SA between 2018 and 2022 before the toppling of his Liberal government by the Peter Malinauskas-led Labor opposition.
He has been a member of the state's Liberal Party and the MP for Dunstan (previously known as Norwood) since 2010.
He was only the second South Australian premier to serve a full term since 1968 and the dismantling of the so-called Playmander system installed by the Liberal-Country Party before World War II to favour rural electorates.
Mr Marshall was a member of the cohort of Australian premiers who held office throughout the COVID-19 pandemic, with the recent departure of Queensland's Annastacia Palazczuk marking the last of the group to continue in the role. Â
"I'm particularly proud of how my government and the broader South Australian community responded to the enormous challenges posed during the COVID-19 pandemic, with swift and decisive action saving thousands of lives and thousands more from the severe disease," Mr Marshall said.
"I send a special thanks to former health and wellbeing minister Stephen Wade, the Commissioner of Police Grant Stevens, and Chief Public Health Officer Nicola Spurrier for their enormous efforts during the pandemic.''
He said he was proud of the economic performance of his government
"We nurtured the space, defence, advanced manufacturing, digital economy, medical research and renewable energy sectors that have left South Australia well placed to enter a new age of prosperity."
Mr Marshall said he intended to formally resign as the Member for Dunstan in coming months.
9°C