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Natural gas is an increasingly attractive fuel for marine applications due to its abundance, lower cost, and reduced CO2, NOx, SOx, and particulate matter (PM) emissions relative to conventional fuels such as diesel. Methane in natural gas is a potent greenhouse gas (GHG) and must be monitored and controlled to minimize GHG emissions. In-use GHG emissions are commonly estimated from emission factors based on steady state engine operation, but these do not consider transient operation which has been noted to affect other pollutants including PM and NOx. This study compares methane emissions from a coastal marine vessel during transient operation to those expected based on steady state emission factors. The exhaust methane concentration from a diesel pilot-ignited, low pressure natural gas-fuelled engine was measured with a wavelength modulation spectroscopy system, during periods of increasing and decreasing engine load (between 3 and 90%).