The senior Congress leader won by over 20,000 votes and defeated his nearest Bharatiya Janata Party rival B.D. Joshi, election officials said.
Rawat was an MP from Uttarakhand before taking over as the chief minister of the hill state and had to become a lawmaker within the stipulated period of six months.
The Dharchula seat was vacated by legislator Harish Dhami to facilitate election of the chief minister as a legislator.
The other two seats - Doiwala and Someshwar - were also held by BJP leaders Ramesh Pokhariyal Nishank and Ajay Tamta who are now party MPs from Haridwar and Almora.
Congress candidates won on both these seats.
Rekha Arya won by over 10,000 votes over her BJP rival in Someshwar, while Hera Singh Bisht won from Doiwala, making it a clean sweep for the Congress.
This comes as a setback for the BJP which made a clean sweep in the Lok Sabha elections held in May, winning all five seats of Almora, Garhwal, Hardwar, Nainital-Udhamsingh Nagar and Tehri Garhwal from Uttarakhand.