The former MP, named only as "Sławomir N.", was detained on suspicion of corruption and operating within an organised crime group.
One of Poland's ex-transport ministers has been arrested in a corruption probe.
The former MP, named only as "Sławomir N.", was detained on suspicion of corruption and operating within an organised crime group, according to Poland's Central Anti-Corruption Bureau.
The suspect had also served as head of Ukraine's highways agency ("Ukravtodor"), the bureau said.
Two other men from Gdansk - a former military commander and a businessman - have also been detained.
The investigation was jointly conducted with the District Prosecutor’s Office in Warsaw, Special Anti-Corruption Prosecutor’s Office in Kyiv, and the National Anti-Corruption Bureau (NABU) of Ukraine.
Ukrainian authorities say that "more than 50 searches" were conducted of Ukravtodor offices and government businesses, as well as homes linked to ex-officials of state agencies in Poland.
The organised crime group is suspected of operating between 2016 and 2019, led by the former head of Ukravtodor who "received illegal benefits" in office.
When Ukrainian companies could not complete road construction contracts, they were offered extensions in exchange for "improper advantage", it is alleged by prosecutors.
"All these cases are related to decisions on the use of funds provided by international financial organizations in the form of loans for the repair of Ukrainian roads," said NABU Director Artem Sytnyk.
Authorities say the total amount of credit funds used by the group to obtain illegal funds exceeds €270 million.
"Maximum measures were taken to mask crimes and illegal origin of funds," added Artem Sytnyk.
The charges against "Sławomir N." could carry a maximum prison sentence of up to 12 years.
In January, a report by NGO Transparency International (TI) found that Poland was one of five EU countries where the misuse of funds is still an issue.
Poland, Hungary, Romania, Croatia and Greece were among the EU's worst in TI's 2019 Corruption Perceptions Index.