Romania's top player needed just 46 minutes to put away her opponent. She dominated the games, winning close to 70 percent of points on her serve. Halep also saved both break points she faced.
"There is no greater joy than winning at home in front of people that care about you," said Halep. "I dedicate this trophy to all the spectators who came to see tennis this week."
Halep had struggled in the semifinal against Vania King of the United States, needing more than two hours to dispatch the American 6-7 (3), 6-4, 6-3.
Heading into Sunday's final, Sevastova held a 3-1 edge in matches against the No. 5-ranked Romanian player.
Halep won the inaugural Bucharest event in 2014. She didn't play in the tournament last year.
Sevastova was playing in her second final of the year. She won the 2010 Estoril Open for her only WTA title and didn't reach a final again until the Wimbledon warm-up at Mallorca last month.