At a ceremony in Paris, where the OECD is headquartered, Kucinskis formally accepted the invitation to join the organization.
Writing on Twitter, Kucinskis said Latvia would "take maximum benefit" from membership of the OECD.
Fellow members, led by Norway, were quick to offer their congratulations.
The signing took place at the OECD’s 2016 Ministerial Council meeting taking place at its Paris headquarters on 1 and 2 June 2016, and preceded on 31 May and 1 June by the OECD Forum.
Both events form part of OECD Week 2016 which brings together government ministers, international experts and key figures from the worlds of academia, business and civil society.
To meet the OECD required membership criteria, Latvia had to institute various reforms of its public administration, financial regulation and business environment.
Kucinskis has insisted that reforms will continue after OECD accession.
President Raimonds Vejonis also expressed his gratitude to Kucinskis and Foreign Minister Edgars Rinkevics: