© 2017 Elsevier Inc. Background Low-Field Magnetic Stimulation (LFMS) is a novel, non-invasive, sub-threshold neuromodulation technique, shown in preliminary studies to have immediate mood elevating effects in both unipolar and bipolar depressed patients. Objective We aimed to assess the antidepressant augmentation effects at 48 h of LFMS administered on two consecutive days compared to sham treatment in treatment resistant depression (TRD) subjects, using the Sequential Parallel Comparison Design (SPCD). Methods Eighty-four eligible subjects with TRD were randomly assigned to double-blind treatment with LFMS 20 min/day for four days, sham treatment 20 min/day for four days, or sham treatment 20 min/day for 2 days followed by LFMS treatment 20 min/day for two days, using the pre-randomization version of the SPCD (randomization 1:1:1). The SPCD analyses used a repeated measures linear modeling approach with maximum likelihood estimation to use all available data, and using a 60–40 weighting of Stage 1 vs. 2 responses, with the primary outcome being measured after 2 and 4 days. Results Both primary and secondary outcome measures consistently showed no differences between LFMS-treated patients and those treated with sham, with the exception of a slight, non-significantly greater improvement than sham in the visual analogue scale (VAS) sad mood on LFMS-treated patients. LFMS treatment was relatively well tolerated. Conclusions We did not observe a significantly greater, rapid efficacy of LFMS compared to sham therapy. Future studies need to examine the possible therapeutic effects of more intensive forms of LFMS, as other forms of neurostimulation typically require longer duration of exposure.