High Bailiff Michael Moyle says those who spit at police can expect to go to prison.
He gave the warning after jailing a Castletown man for four months.
Jason Reed Brown of Douglas Road admitted the offence, but said he had no memory of the incident.
Mr Moyle described it as nasty and unpleasant and told him he was right to expect a prison sentence.
He added he was concerned about the number of people who thought spitting at police was funny, and warned those who did it could expect a stay in a prison cell.