What title is given to Spock as second-in-command of the Enterprise?

The title given to Spock as the Enterprise’s second-in-command is first officer. In The Original Series, Spock combines that command role with his duties as the ship’s science officer.

As first officer, Spock is directly beneath Captain James T. Kirk in the command structure. He advises Kirk, takes command when Kirk is absent or incapacitated, and helps coordinate the bridge crew during crises. His Vulcan logic provides a deliberate counterpoint to Kirk’s instinctive and often risk-taking leadership.

Spock’s rank changes during the series: he is a lieutenant commander in the first season and a commander in later seasons. Rank and position are related but not identical. “First officer” describes his place in the chain of command, while “science officer” describes his specialist assignment.

Scotty is another possible source of confusion. He is generally identified as the Enterprise’s second officer and chief engineer, making him third-in-command. Terms such as yeoman, chief navigator, and fleet captain do not describe Spock’s regular TOS position.