Siam Sinfonietta is a youth orchestra founded in 2010 by Thai/American composer Somtow Sucharitkul, Thai conductor Trisdee na Patalung, and the Bangkok Opera Foundation. It was created in order to provide intensive training for young Thai musicians intending to have a serious career in classical music. The orchestra operates out of the offices of the Bangkok Opera Foundation.

The orchestra came into existence at the first "Bach to the Future: Science through Music" camp and immediately embarked on a series of concerts in Bangkok, particularly concerts for young audiences and concerts that serve to introduce classical music to new audiences. It has toured extensively in Thailand and in its second year plans more international touring as well. Some high-profile events the Siam Sinfonietta has participated in include the "Together We Can" concert to raise awareness of the governor of Bangkok's initiative for reconciliation after the 2010 unrest in downtown Bangkok,and the "Introducing the Symphony" series at Bangkok's Art and Culture Center, populist concert-lectures about music.

The orchestra holds annual auditions and has an entry age of 12, with the cutoff for membership set at 25. A number of its members are also members of full-fledged professional orchestras such as the Siam Philharmonic Orchestra and the Bangkok Symphony Orchestra. It has twice performed in conjunction with the Siam Philharmonic Orchestra: in the Thailand premiere of Mahler's Third Symphony, and at a gala concert in front of  Central World in , Bangkok, the site of the 2010 troubles, as part of an event to promote "Healing through Harmony."

The Sinfonietta receives its funding largely from government grants and private and corporate sponsorship.

On July 8, 2012, the two-year-old orchestra won first place at the Summa Cum Laude International Youth Music Festival in the symphony orchestra category. The next year, the orchestra toured in California and performed at the Young Euro Fest in Berlin, Germany; this was followed in 2014 by performances in Carnegie Hall and another trip to Berlin, Prague and Abu Dhabi under the auspices of the Thai Ministry of Foreign Affairs.

In August 2015 the orchestra performed at the Festival junger Künstler in Bayreuth and at the Thai Festival in Bad Homburg, Germany.

Since then the orchestra has toured Europe and the United States again and again, including performing the European premiere of the opera The Silent Prince in Bayreuth, having a Europe-wide broadcast on Slovak Radio-TV, another outing in Carnegie Hall, and, most recently, at the Oldenburg Film Festival in 2021 where the orchestra trod the red carpet for the world premiere of the film The Maestro, which featured performances of the orchestra both on screen and in the soundtrack. Their work is celebrated world-wide as one of the most innovative and creative young orchestras in the region and the world.