The statement did not mention the Tank Day promotional event.
“The time has come for us to assert our rights and re-establish labour-management relations on an equal footing,” it added.
A Starbucks Korea official told AFP on Sunday, without disclosing how many workers had joined the union, that the company will “continue to engage with the labour union in accordance with the relevant laws and regulations”.
Starbucks Korea employs around 23,000 people.
The labour union is the first of its kind at Starbucks Korea since the chain launched in the country in 1999.
The so-called Tank Day controversy led to a “sharp decline in sales” in the early days of the scandal, according to the operator.
Only a small proportion of the company’s operated stores are unionised in the US and are represented by Starbucks Workers United.
