The 22-year-old Pole cemented her status as the best female player in the world, particularly on clay, with her third title in four years at Roland Garros.
But this was by some distance the hardest of her Grand Slam finals, with unseeded Czech Muchova battling back from a set and 3-0 down to force a decider.
Muchova twice led by a break in that but Swiatek refused to be beaten, eventually prevailing 6-2 5-7 6-4 after two hours and 46 minutes and crouching down on the clay in tears.
She is the first woman since Justine Henin in 2007 to successfully defend her title on the Paris clay and joins Naomi Osaka on four Grand Slam titles - veteran Venus Williams with seven is the only active player to hold more.
— Roland-Garros (@rolandgarros) June 10, 2023
Swiatek joins Osaka and Monica Seles, meanwhile, as the only women in the open era to win each of their first four Slam finals.
Muchova, a 26-year-old ranked 43, produced the performance of her life to beat second seed Aryna Sabalenka in the semi-finals, saving a match point and fighting back from 5-2 down in the deciding set.
But Swiatek had both the experience and a formidable record in finals to bolster her confidence, while Muchova was through to this stage for the first time.
She made a very nervous start and it appeared Swiatek may come up against little resistance but Muchova got on the board in the fourth game and the rest of the first set was competitive.
The Czech showcased the variety she has in her game against Sabalenka, keeping the Australian Open champion off balance with her court craft and willingness to come to the net, and she produced some standout moments here.
There were just too many unforced errors, though, while Swiatek was able to keep her favoured position in the middle of the baseline and dictate with her heavy forehand.
A second break of serve gave Swiatek the opening set, making her the first player, male or female, to win their first seven sets in Grand Slam finals.
When she moved 3-0 ahead again in the second, it seemed number eight was not far away, but Muchova dug in and broke Swiatek for the first time in the fifth game with a brilliant running forehand.
The majority of the crowd was willing her to extend Swiatek further and suddenly it was the Pole feeling the tension, with a double fault handing Muchova the chance to serve for the set.
She could not take it but another shaky game from Swiatek gave her a second chance and this time she made it over the line, clinching her third set point after a stunning all-court rally.
Muchova has struggled badly with injuries during her career and it was only last year that doctors told her she might have to give up the game.
She rode her momentum at the start of the decider by moving into a 2-0 lead as Swiatek threatened to implode but the 22-year-old pulled herself together quickly to level.
They exchanged breaks again in the seventh and eighth games, with Muchova unable to pull away, and Swiatek regained the ascendancy when she fought off another break point to hold for 5-4.
The pressure of serving to stay in the contest proved too much for Muchova, who made three errors before double-faulting on match point in a cruel end to an absorbing final.