The Percy Sladen Trust Expeditions to the Abrolhos Islands were two scientific expeditions, conducted in 1913 and 1915 under the leadership of Professor William John Dakin and funded by the Percy Sladen Trust.
These expeditions conducted extensive research into the natural history of the Houtman Abrolhos, an archipelago in the Indian Ocean off the coast of Western Australia.
The resultant collections and observations were sufficiently important to warrant a second visit, and accordingly a second expedition was funded.
The second expedition lasted for four weeks from October 1915; this time Dakin's companion was A. Cayzer.
The expeditions ultimately contributed substantially to published information on the biota of the archipelago.