Gaiman came away with two gongs, earning 'Best Novelette' for The Truth Is A Cave In The Black Mountains and 'Best Short Story' with The Thing About Cassandra.
Tan, who is best known for illustrating The Arrival and Tales From Outer Suburbia, was named as 'Best Artist', Locus Online reports.
The Locus Awards honour outstanding achievement in the fields of science fiction and fantasy. The annual prize was first handed out in 1971.
Other winners at this year's event include Connie Willis, China Miéville, George R.R. Martin and Gardner Dozois. Tor Books was named as 'Best Publisher'.
The results were announced during the Science Fiction Awards Weekend in Seattle WA, between June 24 and June 26.
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