Sekolah Menengah Sains Kuala Selangor (English: Kuala Selangor Science Secondary School (KUSESS)) is a boarding school, also known as Sekolah Berasrama Penuh (SBP) based in Malaysia. The school was built under the Sixth Malaysia Plan. The school is located in Bandar Melawati, Kuala Selangor which is about 70 kilometers from the nation's capital, Kuala Lumpur. Construction works on the school began on 24 April 1994 and were completed by 1996. The school started operating on 8 September 1996, and its first intake consisted of 210 students. There were only 19 teaching staff and 4 support staff at that time. However, the following year saw a sharp increase in the number of students that were admitted to the school. 187 students gained entry to Form 1 while 165 students were admitted into Form 4. The teaching staff almost tripled from 19 to 53. Although the school began operating in 1996, it wasn't till 27 May 1997 when the school was officiated by the then Deputy Prime Minister-Dato' Seri Anwar Ibrahim
By 1998, the number of students had grown to 810 and the teaching staff had a strength of 63 people. The year also saw the retirement of the school's first principal Tuan Haji Khamis bin Maarof. He was replaced by Tuan Haji Ariffin bin Abdul Rahaman, who left his post at Sekolah Menengah Kadok, Kota Bahru, Kelantan to take the seat as the principal of Sekolah Menengah Sains Kuala Selangor. It was also the year which saw the students of the school sit for Penilaian Menengah Rendah (PMR) and Sijil Pelajaran Malaysia (SPM) examinations for the first time. The results were an overwhelming success as for PMR the school was ranked as one of the top 5 Sekolah Berasrama Penuhs (SBP), while in SPM 147 students out of 148 students obtained first rank.
Today the school has 783 students in 27 classes and they are taught by 75 teachers. The school is respected as one of the best SBP in the country and is the best school in the district in terms of academic achievements.