A harbourmaster (or harbormaster, see spelling differences) is an official responsible for enforcing the regulations of a particular harbour or port, in order to ensure the safety of navigation, the security of the harbour and the correct operation of the port facilities.

Harbourmasters are normally responsible for issuing local safety information sometimes known as notice to mariners.

They may also oversee the maintenance and provision of navigational aids within the port, co-ordinate responses to emergencies, inspect vessels and oversee pilotage services.

The harbourmaster may have legal power to detain, caution or even arrest persons committing an offence within the port or tidal range of the port's responsibilities. An example of this is the team of harbourmasters employed by the Port of London Authority who are empowered to undertake an enforcement role.

Actions that a harbourmaster may investigate include criminal acts, immigration, customs and excise, maritime and river safety and environmental and pollution issues. The police, customs, coastguard or immigration authorities will take over the handling of any offenders or incident once informed by the harbourmaster.

Worldwide there are approximately 3,000 merchant ports and the work of the Harbour Master can vary widely from country to country and from port to port even within the same country.

A harbourmaster may either be a civilian or a commissioned naval officer of any rank.

Historically all harbourmasters were naval officers; even today they must possess prior seafaring knowledge and experience through serving with either a merchant navy or armed navy.