The Blob is a large mass of relatively warm water in the Pacific Ocean off the coast of North America that was first detected in late 2013 and continued to spread throughout 2014 and 2015. It is an example of a marine heatwave. Sea surface temperatures indicated that the Blob persisted into 2016, but it was initially thought to have dissipated later that year.

A marine heatwave is a period of abnormally high ocean temperatures relative to the average seasonal temperature in a particular marine region. Marine heatwaves are caused by a variety of factors, including shorter term weather phenomena such as fronts, intraseasonal events, annual, or decadal (10-year) modes like El Niño events, and longer term changes like climate change. Marine heatwaves can have biological impacts on ecosystems at individual, population, and community levels. MHWs have led to severe biodiversity changes such as coral bleaching, sea star wasting disease, harmful algal blooms, and mass mortality of benthic communities. Unlike heatwaves on land, marine heatwaves can extend for millions of square kilometers, persist for weeks to months or even years, and occur at subsurface levels.