I am a US citizen with a US passport.  She is a Thai citizen, living in Thailand, with a Thai passport.  We are flying from Thailand to Hong Kong, where we will take a cruise to Okinawa and back. We will then return to Thailand for a total of perhaps 10 days spent away from Thailand.  Neither of us should require a visa to enter either country.  But my GF has been reading and found that a double entry visa may be required for Hong Kong.  I called the cruise line and they don't answer such questions, referring me to a visa service company.  They said I should be fine but didn't know about her.  I did some more research and my belief is that a double entry visa is only needed if you stop off in mainland China.  That will not be the case here - Hong Kong to Okinawa and back.  Can anyone verify that I am correct?  Thanks in advance.  

No she will not need a visa.

Whilst my wife hasn't travelled the same route that you have planned, we did travel to Hong Kong, then travel to Macau and then back to Hong Kong before travelling back home to Thailand. Macau is of course a SAR, as is Hong Kong, but we left and were stamped out of Hong Kong, and stamped back in again on our return, without question.

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