The Constitution Defense Monument, built as the Rebellion Suppression Monument and later also known as the Lak Si Monument, was a public monument in Bangkok, Thailand. It was erected in 1936 to commemorate the government's victory over the 1933 Boworadet Rebellion, and stood until its unexplained removal in 2018.

The monument was built by the People's Party government, which came to power with the abolition of absolute monarchy in 1932. After they defeated the rebellion—a royalist coup attempt led by Prince Boworadet—the government built a highway, now Phahonyothin Road, to strengthen its control of Bangkok's northern fringe, where much of the fighting took place. A monument was built there, in a plaza next to the road, to mark the event and honor the men who died fighting for the government.

Over the following decade, the monument was used as a patriotic symbol for the new constitutional state under Prime Minister Plaek Phibunsongkhram, but this narrative was challenged after a 1947 coup d'état brought a resurgence of the royalist faction. The monument subsequently lost most of its political significance, until pro-democracy protest movements revived it as a focal point during the 2010s. In December 2018, the monument was secretly removed overnight without explanation, an act believed to be part of an ongoing effort to erase the architectural legacy of the People's Party government.

The site of the monument is now a traffic circle in Bang Khen district, known as Lak Si or Bang Khen Circle/Roundabout. It forms the meeting point of Chaeng Watthana and Ram Inthra roads with Phahonyothin; its central island is now occupied by the elevated Wat Phra Sri Mahathat station, an interchange station of the Sukhumvit BTS and Pink MRT lines.

In October 1933, following the abolition of absolute monarchy in Thailand the year before, a royalist faction led by Prince Boworadet staged an attempted coup against the Khana Ratsadon (People's Party) government. Fighting ensued between the group and government forces in several locations, including the Bang Khen fields in Bangkok's northern fringe, in the vicinity of Don Mueang Airfield. Heavy fighting broke out, in which the government successfully pushed back and eventually defeated the rebels. Seventeen military and police personnel who had died fighting for the government were hailed as national heroes and given state funerals in Sanam Luang—a treatment usually reserved for royalty.

To strengthen the government's control over Don Mueang Airfield, Prime Minister Phraya Phahonphonphayuhasena proposed in early 1934 that a highway be built to link it to central Bangkok. A monument to honor the seventeen men was also proposed, together with the highway. The proposal was unanimously approved by parliament, and the road, which would later become known as Phahonyothin Road, was built from March to July 1934. The monument, situated in a plaza next to the road, was completed in 1936.

The monument was opened with much fanfare on 15 October 1936, the third anniversary of the battle. The ashes of the 17 men were brought in a military parade from their previous resting places at their affiliated units, and reinterred in the monument in a Buddhist ceremony. The monument was then opened by Prince Aditya Dibabha, the regent, with Luang Phibunsongkhram (Phibun), the Minister of Defense, delivering the opening report.

Over the following years, the monument was the site of annual remembrance ceremonies, which grew into large public affairs following Phibun's rise to the premiership in 1938. Under fascist-leaning Phibun, the monument was leveraged as a patriotic symbol for the new state under the constitutional system. The plaza around the monument was expanded in 1940, and a new "Democracy Temple", another tribute to the constitutional system (now Wat Phra Si Mahathat), was also built near the monument.