Electronic devices may include radio-frequency transceiver circuitry and antenna structures. The antenna structures may include an inverted-F antenna resonating element and an antenna ground that form an inverted-F antenna having first and second antenna ports. The antenna structures may include a slot antenna resonating element. The slot antenna resonating element may serve as a parasitic antenna resonating element for the inverted-F antenna at frequencies in a first communications band and may serve as a slot antenna at frequencies in a second communications band. The slot antenna may be directly fed using a third antenna port. An adjustable capacitor may be coupled to the first port to tune the inverted-F antenna. The inverted-F antenna may also be tuned using an adjustable capacitor bridging the slot antenna resonating element.