This study presents the first comprehensive systematic revision of the sponge family Tetillidae in the temperate north-east Pacific. The findings reveal a previously unrecognized clade of Tetilla species that inform the family’s generic classification, including a common California species whose identity has long been uncertain. Remarkably, eight species were discovered from only 11 samples from the Aleutian Islands of Alaska, suggesting that these islands harbour considerable undiscovered sponge diversity. I also report the first likely case of species’ introduction in the family: an Asian species, probably Tetilla japonica, appears to have been introduced to Southern California. In total, this work refines our understanding of nine previously described species and formally describes eight new species as Craniella amlia sp. nov., Craniella shemya sp. nov., Craniella columbiana sp. nov., Craniella rocheta sp. nov., Craniella uniiguni sp. nov., Craniella vermisigma sp. nov., Tetilla losangelensis sp. nov., and Tetilla vancouverensis sp. nov.. Multi-locus DNA sequence data are presented for 13 species, including several low-coverage mitochondrial genomes and complete ribosomal sequences assembled de novo from Illumina sequencing. Together, these results significantly advance our understanding of global tetillid diversity, systematics, and the biogeography of sponge diversity in the north-east Pacific.