Her decision was revealed in her Daily Mail column and follows 12 months of deliberation, coming just before Reform's annual conference where she will deliver the opening speech.
Dorries stated her belief that Reform leader Nigel Farage has the answers and the will to address critical issues facing the country.
She cited alignment with Farage on key policies including law and order, immigration, the need for public spending cuts, boosting growth, and supporting Ukraine.
The defection has prompted reactions from other political parties, including the Liberal Democrats and the Green Party.