In both 2012 and 2013, by contrast, more than 80% of the Grade 12 students who took the exam passed it, education officials said on Friday.
The education ministry said that of the 89,937 students who took exam on Aug 4, only 23,126 of them, or 25.72% passed.
Ministry spokesman Ros Saline said the low percentage reflected the government's efforts to curb cheating and its insistence on strict adherence to exam procedures as it pushes forward with reforming the education system.
Last year, 86.77% students who took the national exam passed, as did 86.48% of those who took it a year earlier, according to Saline.
Cheating had until recently been the norm in Cambodia, with students bribing exam supervisors to take answer sheets into the classroom or using their mobile phones to cheat under the noses of bribed proctors.