The package arrived in the morning post at his constituency office in Lurgan, Co Armagh, said the RUC. The device was "crude but capable of exploding", said a spokesman.
A number of shops and businesses close to the Queen Street constituency office were evacuated while army bomb disposal experts made the device safe. It was the latest in a series of similar devices sent to leading Ulster politicians.
Last week another was defused after being sent to another Ulster Unionist MP, Jeffrey Donaldson, at his office at the Lisburn Council headquarters in Hillsborough, Co Down.
Similar devices have been made safe after being delivered to UK Unionist leader Robert McCartney, MP, at both his Bangor, constituency office and his office in Belfast. No organisation has claimed responsibility for any of the attacks.