TAIPEI—HTC Corp. on Monday posted its lowest quarterly profit since 2006, as the company faced further pressure from rivals in the lucrative market for smartphones.

Taiwan-based HTC, which has been losing ground against Apple Inc. and Samsung Electronics Co. in the smartphone market, said its third-quarter net profit totaled 3.9 billion New Taiwan dollars ($133 million), off 79% from NT$18.64 billion a year earlier.

Revenue...