The only goal of the game was scored in the second half when Taariq Fielies found the back of the net with 20 minutes of the contest remaining.
The three points cemented Amazulu's seventh position in the standings, four clear of eighth-placed Kaizer Chiefs.
The defeat means the Natal Rich Boys remain in danger of relegation as they find themselves only two points clear of SuperSport United who sit in the relegation playoff place.
Amazulu had the first opportunity of the game when Nkosikhona Radebe tested goalkeeper Ian Otieno, who made a routine save in the 13th minute.
Usuthu eventually got the winner in the 69th minute when Otieno parried the ball into the path of Fielies, who made no mistake as the defender fired home from close range.
Richards Bay looked to snatch a late equalising goal, however, AmaZulu stood firm at the back to claim a narrow 1-0 victory.
The Betway Premiership delivered drama, with Marumo Gallants and SuperSport United trading blows in a pulsating draw, while Golden Arrows and Chippa United battled to a tense goalless stalemate.
Orlando Pirates kept their Betway Premiership title hopes alive with a gritty 1-0 victory over Polokwane City at the Old Peter Mokaba Stadium on Saturday, thanks to a second-half finish from defender Nkosinathi Sibisi.
AmaZulu head coach Arthur Zwane believes his side could have been among the top three in the Betway Premiership standings if they had capitalized on earlier opportunities this season.
The Betway Premiership heats up this Saturday as Orlando Pirates prepare to clash with Polokwane City at the Old Peter Mokaba Stadium. Kick-off is set for 15h00.
