Police confirmed breaking reports about the incident at Briggate in Brighouse at 1.07pm.
The guard was attacked with ammonia before being robbed of his cash box and is now being treated in hospital.
A West Yorkshire Police spokesman said: "Officers attended to find a security guard had been attacked outside a bank by a man who then ran off with the cash box the guard was holding.
"It is believed the victim had an ammonia type substance sprayed in his face, and he is currently receiving medical treatment at Calderdale Royal Infirmary.
"Enquiries are ongoing in Brighouse town centre by Calderdale CID."
Anyone who witnessed the incident is asked to contact Calderdale CID on 101 referencing police log 870 of 23/01.