Latest figures show a rise of nearly 35% in crimes of indecency between April and July this year compared with the same period in 2015.
Ch Supt Gary Ritchie said the increase had not been unexpected.
He said sex offences were the most under-reported crimes and they were working to address that.
A total of 112 indecency offences were reported in the months covered by the latest report to Dumfries and Galloway Council - it included 37 sexual assaults and 16 rapes.
It compared with a total of 83 over the same period last year.
Ch Supt Ritchie said: "It is what we expected and, in actual fact, what we hoped to see.
"The recent Scottish crime and justice survey has indicated that rape and sexual crime is the most under-reported crime in any category.
"An estimation carried out by the Scottish government suggests that only 40% are reported.
"So when we see an increase in that, that is encouraging for us."
Police in Dumfries and Galloway have said a rise in reports of sex crimes is "encouraging" as it shows victims are more confident about coming forward.