Sempere joins from English Championship side Norttingham.
The 27-year-old Spanish international can play at full-back and fly-half and has European experience from his time at Olympus Rugby XV Madrid and El Salvador.
"Our campaigns in the Premiership and in Europe have shown us the value of having a strong all-round squad, and with the World Cup coinciding with the start of next season we need players who can help us compete throughout next season," said Saints director of rugby Jim Mallinder.
"James and Craig have shown their quality in the Championship and in the games they have played against us in the last year, and we were impressed by what they can bring, not just in terms of playing ability but desire to succeed in a Saints shirt.
"We look forward to welcoming them to Franklin's Gardens during the summer."