A spokeswoman for South Wales Fire and Rescue service said a boy, described as under 10-years-old, threw a plastic container of horse manure at the engine.
The appliance was undamaged and the four crew members were reported as unharmed.
Alison Kibblewhite, head of risk reduction at South Wales Fire and Rescue, said: “We have a fire crime department to look at the evidence and have CCTV on all our fire engines.
She added it was the fourth attack on fire crews from South Wales Fire and Rescue this year, after 16 attacks were recorded last year.
“We’ve had two prosecutions this year following incidents of abuse. Objects are sometimes thrown or people shout verbal abuse.”
FIREFIGHTERS were attacked by people throwing manure at them when they tried to rescue a trapped horse last night.
Firecrews were called to the incident at Peterstone on Wentloog Road at 9.55pm.