Traditionally, fracture criteria have been established based on a crack-tip material length scale which famously is called the critical distance. This parameter is considered to be independent of the mode mixity and measured under pure mode I loading conditions as a material parameter. However, recent studies show this parameter may considerably change between the fracture modes depending on the mode mixity. The mode-dependent critical distance, called the effective critical distance, is found to be a reasonable approach when implemented into a strain-based framework. In this paper, the concept of effective critical distance (ECD) is combined with the maximum tangential strain energy density criterion (MTSED), and a novel mixed-mode I/II/III brittle fracture criterion, called ECD-MTSED, is suggested. The new criterion is used to predict the experimental data reported in the literature for the mixed-mode I/II/III fracture toughness and fracture angles along with four other criteria – namely, maximum tangential stress (MTS), maximum principal stress (MPS), strain energy density (SED), and maximum principal strain using an effective critical distance (ECD-SN). The results not only showed the superiority of the suggested criterion but also accentuated the necessity of employing effective critical distance in the three-dimensional brittle fracture theories.