He has held the Otaki seat since 2008 and will have served 15 years in Parliament by next year's General Election.
During his time in government, he was the Minister of Primary Industries, Internal Affairs, Immigration, Veterans' Affairs, Civil Defence and Racing.
"During this time I've worked with six mayors, seen huge growth and development in the electorate and had wonderful staff supporting me at my offices in Paraparaumu and Levin."
Guy is the third National MP in the space of just over a month to announce they won't be seeking re-election next year.
Late last month, Amy Adams – National's finance spokeswoman and former Justice Minister also stood down. She said she wanted to spend more time with her family.
An hour later, Wairarapa MP Alastair Scott announced he would also stand down ahead of the 2020 election.
In his statement, Guy said his number one priority has been to support the people of Kāpiti and Horowhenua.
"I have, and will continue to be a strong supporter of rural communities, especially as minister through adverse events like the dairy downturn, prolonged North Canterbury drought, earthquakes and floods.
"I advocated hard for water-storage projects and helped secure funding for a variety of projects including Central Plains stages one and two."
He thanked his family for their support and said he is excited about what the future may hold for Horowhenua and Kāpiti.
"I will always advocate for the region I'm so proud to call home."
