Firefighters were alerted at 4.40pm to a fire in a field, on Horsham Road.
West Sussex Fire and Rescue Service confirmed it was an accidental ignition, likely caused by the heat.
A spokesperson said: "Crews from Littlehampton, Worthing, Shoreham and Horsham [found] approximately three acres of cut wheat, stubble and baled straw alight.
"There was 10,000 square metres damage."
Crews had left the scene by 7.30pm.
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