Botswana's Montsho, aiming to become only the second woman to win back-to-back world 400 golds, was left to despair the fact she did not dip at the finish, with just 0.004 seconds separating the two rivals who were both given a time of 49.41 seconds.
The pair endured an anxious wait before the result came up on the big screen at the half-empty Luzhniki stadium and a beaming smile broke out across Ohuruogu's face as her name flashed up first while Montsho was left staring in disbelief.
Russia's Antonina Krivoshapka took the bronze in 49.78s.
"I feel like I'm in a dream, I can't believe it," Ohuruogu, whose time was a British record, told the BBC.
"I just thank god that everything came together today. It has been really tough. It feels really surreal."
Ohuruogu, World champion in 2007 and Olympic gold medallist in 2008, is known as a big race performer who often leaves her attack until the final few metres and the 29-year-old did not disappoint the British fans noisily screaming her on.
She was fourth coming off the final bend but powered down the home straight to catch Montsho on the line.
Image: Christine Ohuruogu
Photograph: Cameron Spencer/Getty Images
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