Zendesk agreed in late October to buy Momentive, the parent of online survey portal SurveyMonkey, in an all-stock deal, which at the time was valued at nearly $4 billion.
"We are gravely concerned about the strategic, tactical, and financial rationale of this proposed deal," Janus Henderson Investors said in a proxy statement.
"We think the company should instead renew its focus on executing on the (Zendesk) standalone business."
The activist investor did not disclose the size of its stake in Zendesk, but said it was a large, multi-year shareholder in the company.
Refinitiv data showed Janus Henderson held a 4.94% stake in Zendesk, which had a market capitalization of $12.21 billion as of stock's last close.
Zendesk, a customer service software vendor, has seen a surge in business during the pandemic as its enterprise customers accelerate digital transformation.
In November, Jana Partners urged Zendesk to abandon its deal for Momentive.
Shares of Zendesk were up marginally in trading after the bell, while those of Momentive were flat.
(Reporting by Chavi Mehta in Bengaluru; Editing by Shailesh Kuber)
