<p>The effect of a tuned liquid damper (TLD) consisting of a partially liquid-filled rectangular tank attached with a vertical porous baffle, on the motion responses of a floating pontoon in waves, has been systematically investigated. The hydrodynamic problem of a floating pontoon in waves and sloshing dynamics under the swaying and rolling excitation are solved independently by a matched eigenfunction expansion method (MEEM). The vertical porous baffle for additional damping of the TLD is modeled using an equivalent linearized quadratic velocity model. The motion responses (sway, heave, and roll) of a floating pontoon, coupled with the TLD, are validated by self-conducted experiments at a two-dimensional wave tank. The TLD with no-baffle creates a double-peaked roll response amplitude operator (RAO) by adding a newly emerging resonance peak at the first sloshing-mode natural frequency. The installation of a vertical porous baffle removes the sloshing-mode resonance peak due to increased damping and shifts the remaining resonance peak to the low-frequency region. It is concluded that the TLD system with a porous baffle does not contribute to reducing the sway and heave motion significantly and is very effective in reducing the roll motion response of a floating pontoon, if the appropriate TLD geometry, baffle porosity, and immersion depth of a baffle are selected.</p>