Liquid supply apparatus, printing system, and printing apparatus

A liquid supply apparatus configured to supply a liquid to a printing unit, comprising:

• a tray on which a flexible retaining container that retains a liquid is configured to be placed;
• and
• a tray holder configured to hold the tray and to allow insertion of the tray in a first direction,
• wherein the retaining container includes a supply port configured to supply a liquid to the printing unit and an inlet coupled to the supply port and configured to take in a liquid from the retaining container, and
• the tray being provided with a concave shape formed on a bottom surface of the tray, the concave shape overlapping the inlet from a viewpoint in an up-down direction in a state in which the tray is inserted in the tray holder.