Poroshenko's press service said on October 9 that the bill adopted by the parliament on September 16 has been signed by the president.
Under the law, up to one million public servants, including cabinet ministers will be screened for loyalty to root out the corrupt practices of previous pro-Russian President Viktor Yanukovych's administration.
The law is expected to purge officials linked to the pro-Russian separatists in Ukraine's east and other Russian structures.
Officials unable to explain their sources of income and assets will be banned from public office for five to 10 years.
Poroshenko said he signed the bill with the aim of "restoring confidence in [Ukraine's] authorities."
Ukrainian Interior Minister Arsen Avakov said earlier on October 9 that he would start the lustration process within his ministry as soon as the law was signed by Poroshenko.