Potassium (K+) is an essential macronutrient involved in regulating plant growth and development. K+ homeostasis in plant cells is modulated to facilitate plant adaptation to K+-deficiency stress. Argonaute1 (AGO1) interacts with miR-168a to modulate the small RNA regulatory pathway in tomatoes. However, the roles of AGO1 and miR-168a in modulating K+ deficiency stress in tomatoes have not been elucidated. Accordingly, in this study, we examined the interactions between AGO1 and miR-168a in mediating low-K+ stress in tomato plants. SlmiR-168a and its target SlAGO1 were differentially expressed between low-K+ tolerant JZ34 and low-K+ sensitive JZ18 tomato plants. Transgenic tomato plants constitutively expressing SlmiR-168a and rSlAGO1 (SlmiR-168a-resistant) showed different root hair development, leaf phenotypes, and K+ contents in roots under K+-deficiency stress. Sequencing analyses showed that 446 microRNAs (miRNAs) and 541 miRNAs were differentially expressed in 35S:SlmiR-168a compared with wild-type (WT) tomatoes and in 35S:rSlAGO1 compared with WT tomatoes, respectively. Twelve miRNA/mRNA pairs were identified, and the root growth and cytokinin (CTK)/abscisic acid (ABA) pathways were shown to be involved in SlmiR-168a-mediated SlAGO1 regulatory network in response to K+-deficiency stress. Thus, SlAGO1 regulated by SlmiR-168a may influence downstream miRNA pathways in response to low-K+ stress though modulating root growth and CTK/ABA pathways.