Janine Gibson, a former senior editor at The Guardian who left the newspaper recently after being passed over for its top editor’s post, will join BuzzFeed as the editor in chief of its British operation.

Ms. Gibson, 42, said in an interview that she would start at the end of summer and would oversee an expansion of BuzzFeed’s news staff in Britain, adding more than a dozen employees to a newsroom that now has about 45.

Ms. Gibson had been at The Guardian for 17 years. She oversaw the start of its American news operation and supervised its articles based on leaked material from the former National Security Agency contractor Edward J. Snowden, which secured the newspaper its first Pulitzer Prize.

But in March she was overlooked for the position of editor in chief of The Guardian and she announced last month that she would leave.