Tillerson announced a department-wide survey and interviews intended to determine how foreign service officers and civil servants are "going about completing the Department of State's mission."
The survey announcement comes as the top US diplomat is scheduled on Wednesday to address State Department staff. The Associated Press first reported on the letter.
"One of the great challenges and opportunities for State Department and USAID staff is deciding how to confront changing conditions in every corner of the world," Tillerson wrote in the letter, a copy of which was obtained by CNN. "We need the benefit of your natural and well-developed skills to adapt to the changes that we will face over the next twenty years."
The agency will interview 300 randomly chosen employees representative of a cross-section of the department's workforce. The survey will be sent to some contractors, employed family members of State Department staff and local staff.
The Trump administration plans to cut 2,300 jobs at the State Department as it whittles the budget to just over $50 billion. At the same time, it plans to boost spending for the Pentagon -- plans that will ultimately have to be approved by Congress, which controls the purse strings.
The proposed cuts have drawn protests from both parties in Congress and dozens of retired military generals.
Nicholas Burns, the undersecretary of state during President George W. Bush's administration, tweeted that the proposed cuts would "weaken America and cripple the foreign service. Shocking."
Tillerson said in the letter that he has "no pre-conceived notions about how the Department or USAID should be organized for the future."
During the presidential transition, Trump's team asked the State Department to provide it with a list of all programs
and activities intended to promote gender equality. The department, like the UN and many other foreign countries, has seen improvements for women and girls as a cornerstone of international development efforts.
The Trump transition team sent other agencies similar questionnaires about issues the President has expressed doubt about, including climate change. The transition team's questions raised fears that these programs would eventually be caught in the cross-hairs.
Tillerson telegraphed the looming cuts on his first day at the State Department, telling staff at the welcome ceremony that "we cannot sustain ineffective traditions over optimal outcomes."
His first move -- while he was on his first overseas trip to Germany -- was to have his aides tell most of the management and staff in the offices responsible for running the department that they were being reassigned.
The news sent shock waves
through the agency and left career officials on edge. They would have acted as the connective tissue between Tillerson, a government novice, and policy experts throughout the building.
Almost three months later, those management vacancies remain unfilled.