# [MATL], 5 bytes

    t:Ysm

[Try it online!]

[MATL]: https://github.com/lmendo/MATL
[Try it online!]: https://tio.run/nexus/matl#@19iFVmc@/@/oQEA "MATL – TIO Nexus"

Explanation:

    t       % Duplicate input
     :      % Range(1, input)
      Ys    % Cumulative sum. This will push the first *n* triangular numbers
        m   % ismember. Pushes true if the input is contained within the array we just pushed