Is a ball that lands on the line in or out in beach tennis?

In. A ball landing on any boundary line is considered good (in bounds) per ITF beach tennis rules. This applies to all lines: baseline, sideline, service box lines.

Full Explanation

Per ITF Beach Tennis Rules, all boundary lines are considered part of the court. A ball that touches any part of a court line -- whether the baseline, a sideline, the service box boundary, or the center service line -- is considered in bounds and is a good shot. This is consistent with standard tennis line call rules. Only a ball that lands entirely outside a boundary line (no contact with the line at all) is called out. On a serve, a ball landing on the service box lines (including the far baseline and the sideline of the service box) is a good serve. Line calls in beach tennis are made by the players themselves in recreational play, or by umpires and line judges in official tournaments.

Ruleset: ITF  |  Rule Reference: ITF Beach Tennis Rules, Rule 11 (Ball in Play)