Lithuania was on May 3 formally invited to join the OECD convention and signed its accession agreement in late May.
The agreement was ratified by the parliament last week and is to be deposited with the French government.
Lithuania opened its membership negotiations three years ago and officially finalized the talks in April.
The OECD membership will initially cost the country around 2.8 million euros per year, with the fee expected to go down to 1.8 million euros from 2022.
Lithuania lagged behind its EU neighbors in the accession process: Poland joined the organization in 1996, Estonia in 2010, and Latvia in 2016.
Proponents say that OECD membership will boost Lithuania's reputation among investors and will give it access to the organization's expertise.