They said the organisation will give staff greater power to voice concerns about discriminatory work practices at the firm and how it handles issues like online hate speech.
The move follows walkouts and other actions by staff in recent years.
Google said it would "continue engaging directly with all our employees".
"We've always worked hard to create a supportive and rewarding workplace for our workforce," Kara Silverstein, director of people operations, said in a statement.
"Of course our employees have protected labour rights that we support. But as we've always done, we'll continue engaging directly with all our employees".
The announcement of the Alphabet Workers Union comes weeks after Google's firing of a high-profile black artificial intelligence and ethics researcher generated uproar.
The US National Labor Relations Board also recently ruled the firm had unlawfully fired employees for attempting to organise a union.
Staff have also mobilised against the firm's "Project Maven" work with the Department of Defense and the company's handling of sexual harassment complaints.
"This union builds upon years of courageous organizing by Google workers," Nicki Anselmo, program manager, said in the announcement.
"From fighting the 'real names' policy, to opposing Project Maven, to protesting the egregious, multi-million dollar payouts that have been given to executives who've committed sexual harassment, we've seen first-hand that Alphabet responds when we act collectively.
"Our new union provides a sustainable structure to ensure that our shared values as Alphabet employees are respected even after the headlines fade."
The group was organised by software engineers but is open to all ranks at the company's US and Canadian workforce, including temporary workers and contractors.
It is affiliated with the larger labour group, Communication Workers of America, but is not seeking formal recognition from the federal government, limiting its bargaining power.
It represents a small fraction of Alphabet's workforce, which includes more than 130,000 people as of September and roughly as many contractors, vendors and temporary staff.
Members who join will contribute about 1% of their compensation to the effort.
"We want Alphabet to be a company where workers have a meaningful say in decisions that affect us and the societies we live in," organisers wrote on Twitter.