It said a consultative ballot on industrial action will be held after failing to see progress from a meeting with Health Secretary Jeane Freeman more than a month ago.
The union's "pay up now" campaign is calling for the three-year NHS pay deal to be renegotiated in light of the pandemic and extra costs incurred by healthcare workers.
Discussions with the Health Secretary have been ongoing for several months.
Unison's head of health Willie Duffy said: "Unison members are disappointed and angry that the Scottish Government has failed to deliver on pay for NHS workers this year - despite assurances from both the Scottish Health Secretary and the First Minister.
"They are happy to praise our NHS workers and offer warm words on pay but are unable to back that up by putting money into the pockets of our hard-working and devoted NHS staff.
"The last thing our members want to do at this time is disrupt NHS services but more than a month on from our talks with Health Secretary Jeane Freeman we are no further forward.
"The Scottish Government has failed to deliver on pay for our NHS workers and we feel we have been left with no other choice."
Tom Waterson, chairman of Unison Scotland's health committee, said: "Our dedicated NHS staff have suffered real-term pay cuts over the last decade with significant extra costs over this pandemic.
"This includes extra childcare costs because they are working extra shifts; extra transport costs because of restricted public transport; extra costs because they have being isolating from family, or even extra laundry costs and buying personal PPE.
"Our politicians are quick to praise the hard work and dedication of our NHS workers who have gone above and beyond during this pandemic but unfortunately clapping doesn't pay their bills."
Ms Freeman previously said talks with the union and hearing from NHS staff were "vital" to understanding what the workers are going through in dealing with the pandemic and "reaching a deal that reflects and recognises their efforts".
She added: "I have been repeatedly clear that we are considering several options for this which take into account the efforts of NHS staff during the pandemic, and have not ruled out reopening the final year of the current three-year pay deal as part of that 2021-22 deal."
A Scottish Government spokeswoman said: "The Scottish Government remains absolutely committed to delivering the fairest pay deal possible for health staff.
"It is disappointing to hear that Unison intend to proceed with a vote on a consultative ballot before negotiations have even begun.
"Talks have not broken down and we remain in active discussions with all parties, and have been meeting frequently to conclude a negotiating mandate in order that a formal pay negotiation can begin.
"The Cabinet Secretary for Health has reaffirmed her commitment to expedite negotiations and to consider bringing forward the agreed pay settlement to an earlier juncture if it is possible to conclude negotiations before the start of the new financial year."