Williams was back giving the crowd a one-arm raised victory twirl after reaching a 10th Wimbledon final after her opponent swiped a lob behind the baseline.
Kerber is bidding to become the first German woman to win Wimbledon was Steffi Graf in 1996.
For the second time in three years, Williams will face Angelique Kerber for the title.
After Kerber had her serve broken in the second set, Ostapenko powered back to take another game, but Kerber was able to complete the sweep when Ostapenko fired two serves back into the net and returned the final volley outside the lines on match point.
"These are the matches I was working for as a young kid", Kerber said, "and to stand here again in the final at Wimbledon is great".
Ostapenko, 21, had admitted she cracked under the strain of defending the French Open title when she lost in the first round last month.
And the first Latvian woman to make the Wimbledon semi-finals was paying the price for more rash mistakes as the match slipped away. She won the grass-court tournament the last two times she played it, in 2015 and 2016, before missing it a year ago while pregnant.
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England opener Jason Roy took to Twitter and wrote,"Did us proud @ England ". He sets the tone, he brought this team together.
Kerber is now back in a Grand Slam title match for the first time since beating Karolina Pliskova in the 2016 U.S. Open final - having begun that year by stunning Serena Williams in the final of the Australian Open.
The 23-time major victor is just one title short of matching the legendary Australian's haul of Slam title and she will contest the final at the All England Club for the chance to match her.
She is on a 19-match winning streak at the All England Club, with her last defeat coming against France's Alize Cornet in the 2014 third round.
But she can move level with Court on 24 Slams won if she beats Angelique Kerber on Saturday afternoon.
The final tally told the story: Ostapenko had far more winners, 30-10, but also far more unforced errors, 36-7.
She was, instead, going to employ spectacular defense and solid, steady play, while letting her opponent, Jelena Ostapenko, be the one to determine the outcomes of almost every point.