Wolff has been Williams’s development driver for the past three seasons, a position that included participating in Friday practice at the British and German Grands Prix this year.
Her new position as official test driver for 2015 guarantees a further two Friday practice appearances plus two tests, alongside extensive simulator duties.
“This is another step in the right direction for me,” said Wolff.
“I am delighted Williams are recognising my progression, hard work and that it is performance that counts.”
Deputy team principal Claire Williams said the quality of Wolff’s feedback and technical nous had become a major asset to Williams.
“She continues to impress us with her strong technical knowledge, the feedback she delivers and the performance when she drives the car both on the track and in the simulator,” said Williams.
“She has steadily increased her time behind the wheel since she joined us in April 2012 and her appointment as official test driver was a natural progression.”
Williams will recruit a new driver to take on Wolff’s previous development role.
It has retained Valtteri Bottas and Felipe Massa as its race drivers for 2015.