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If you are in a specific forum and seeing the list of threads, on the right is the "Thread Tools" dropdown and it is an option in there.
Not something I use as I tend to just use the "New Posts" option at the top and then the list of returned threads tells you where you have read up to, so not sure where else that option may have been.
Right near the bottom, just above the "Whats going on" section there is a Mark Forums Read link in the middle. Not the most obvious or useful place for the link.