The top-ranked and top-seeded Williams won the hard-court event for the third time.
Williams will be No. 1 for the 200th week in her career when the rankings are announced Monday. She is fifth on the career list, nine weeks behind fourth-place Martina Hingis.
Williams beat the left-handed Kerber for the fourth straight time and fifth time in six overall meetings. All of their matches have been in straight sets.
Kerber, 0-4 in finals this season, won five straight games to go up 5-1 and was serving for the first set at 5-2. Williams saved two set points and went on a five-game run of her own.
The eighth-ranked Kerber forced a tiebreaker, where Williams took charge with five straight points to open.
The second-seeded Raonic produced serves topping 140 mph and broke the 13th-seeded Pospisil four times at the hard-court tuneup for the
Raonic, a semifinalist at Wimbledon last month, earned $316,400 for the victory and his ranking will rise one spot Monday to No. 6, matching his career high.
The ATP said it was the first time two men from Canada played each other in a tour final in the Open era, which began in 1968, and red-and-white maple leaf flags dotted the stands on the outskirts of Washington.
Two-time major champion