Knight, 27, appeared to trip before she made it to the first hurdle in her race and ended up tumbling into it and was left sprawled on the floor.
Knight will get a chance to run again in the 4x400m relay later in the games, but her individual medal quest lasted only seconds.
She appeared distraught after picking herself up off the track, and covered her face with her hands as the tears seemed to flow.
Her crash came after a false start in the heat, which seemed to be caused by the echo of the starting gun in the empty stadium.
Knight has temporarily given up her job as a primary school teacher in Surrey to prepare for the Olympics.
“She will be shattered with that,” Tamsyn Manou said in commentary for Australia’s Channel 7.
“She would have worked so hard to get to the Olympics. I don’t know whether she had a problem with her spikes, it looked like she slipped before she took off.”
There was an immediate outpouring of support for Knight on social media after the incident.
“Feel so bad for Jessie Knight what an absolute stinker, imagine training all those years just for your ankle to roll before the first hurdle,” tweeted @miedemamadness.
“STOP THE OLYMPICS!! We have to give Jessie Knight another shot. That was absolutely heartbreaking,” tweeted @mollykord.
And @therealannawebb tweeted: “Oh, Jessie Knight. That was heartbreaking.”