The English boxer won the vacant World Boxing Association (WBA) bantamweight title after Na Rachawat was halted with a minute of the tenth round remaining on the undercard of Carl Froch-George Groves at Wembley Stadium.
McDonnell, 28, had assumed control of the fight and uncorked a beautiful counter left hook on the ropes that sunk Na Rachawat.
The Thai got to his feet at the count of seven but his legs betrayed him and he stumbled into the ring post before the fight was waved off.
McDonnell, from Doncaster, suffered a setback early on when he was cut above the left eye by a clash of heads in the third round.
Na Rachawat had never fought outside of his native Thailand but that did not seem to bother him as he pressed back McDonnell in the first three rounds.
But McDonnell, who gets married in Mexico in late June, did better in the fourth when he snapped back Na Rachawat's head with a right uppercut.
McDonnell took control of the sixth, boxing neatly and landing a sweet left hook that Na Rachawat absorbed.
Na Rachawat kept bouncing forward but he began being hit with more regularity, although that did not stop him raising his arms at the end of most rounds.
McDonnell, who was stripped of the IBF version of the world title last October for failing to meet mandatory challenger Vusi Malinga, ended the fight with an excellently-time punch that landed flush on the jaw. AFP