Nandi Hills MP Alfred Keter has become the fifth MP to lose his seat following a successful petition by independent candidate Benard Kibor Kitur.
The High Court in Eldoret on Thursday found that the August 8 poll that he won was marred by malpractices and irregularities.
Justice George Kimondo said the election was not held in a free and fair environment.
The Independent Electoral and Boundaries Commission declared Mr Keter the winner of the election with 28,923 votes against Mr Kitur's 13,872.
Mr Keter had asked the court to strike out the petition on grounds it was defective and lacked merit.
The legislator also wanted affidavits filed by a new petitioner in the case to be expunged from the court records but failed.
The petition was initially lodged by Mr Robert Kemei who withdrew and was substituted by Mr Kitur, who emerged second in the election.
Judge Kimondo also said he was satisfied that the MP campaigned outside the time stipulated in the Elections Act.
He said the fact that Mr Keter had confirmed that he was at Ainapngetuny village, Labuiywo trading centre, Kapkoros and Stima School in Oll’essos 48 hours to the poll was a clear indication that he was campaigning.
"This court finds sense in evidence produced by the petitioner that Keter campaigned when IEBC had officially closed campaign period as per the election laws," the judge ruled, though the legislator denied he was campaigning.
"Under the Electoral Code candidates were obligated to implement and obligate orders and directions of IEBC. In this situation I find that the first respondent did not adhere to the regulations of IEBC," Mr Kimondo said.
He added: "Granted those circumstances, a declaration is hereby issued that the election held on August 8, 2017 for the Member of National Assembly of Nandi Hills was not free and fair; Keter was not validly elected as Nandi Hills Member of National Assembly."
Mr Keter termed the judgment a political decision that is meant to gag him from speaking against the government.
He was accompanied by MPs Silas Tiren (Moiben) and Johana Ng’eno (Emurua Dikirr).
Other legislators whose victories have been nullified are: Chris Karani (Ugenya), Kangogo Bowen (Marakwet East), Wanjiku Kibe (Gatundu North) and Lamu West's Stanley Muthama.