Synrem, the eldest son of late former Assembly Speaker and UDP supremo Donkupar Roy Dkhar, defeated his nearest rival Grace Mary Kharpuri, an Independent, by 6,221 votes.
The by-election was necessitated by death of Roy on July 21.
Synrem secured 12,002 votes and Kharpuri, the lone wonam in the fray and an elected member of the Khasi Hills Autonomous District Council, bagged 5,781 votes.
As many as 188 voters opeted for the NOTA (None of the above) option. The voting on Monday saw 84.56 per cent of 29,542 electorate casting their votes.
Synrem also surpassed the cumulative winning margin of Roy over last seven terms from the Shella constituency.
The National People's Party, which is leading the Meghalaya Democratic Alliance (MDA) coalition government, and the Hill State People's Democratic Party supported Synrem.
The UDP is a key partner of the ruling MDA coalition government.
The Bharatiya Janata Party and the People's Democratic Front, other key partners in the MDA coalition government, had also fielded Joshua Warjri and Mosjo Rock F. Wanswett, respectively.
while Warjri polled 742 votes, Wanswett 4,327 votes. Congress nominee Batyngshai Laitmon Ryngnga secured 1,715 votes. Former HSPDP leader Komen Laitmon could get only 444 votes.
With this victory, the UDP has raised its seat to 7 in the 60-member House.
( With inputs from IANS )