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The 17-year-old Australian established a body-length lead inside the first 100m and simply maintained her relentless pace through the middle stages before she kicked away in the final 200 metres to clock eight minutes, 20.02 seconds.
Compatriot Jessica Ashwood finished second in 8:27.60, with Kiah Melverton storming home in the final 50 metres to claim bronze and give the host nation a clean sweep of medals.
Welsh swimmer Jazmin Carlin, the Glasgow champion and Rio Olympics silver medallist behind the imperious American Katie Ledecky, finished sixth, more than 17 seconds behind Titmus.