Titanium is commonly used as a biomaterial for dental implants. In this study, we investigated the antibacterial properties of titanium samples following treatment with a non-thermal atmospheric pressure plasma jet (NTAPPJ) on bacteria with two different cell wall structures, including gram-positive and gram-negative bacteria. The hydrophilicity and surface energy of titanium surfaces were significantly increased after NTAPPJ treatment without altering topographical features. Changes in the chemical composition and reductive potential were observed on the NTAPPJ-treated titanium surfaces. The adhesion and biofilm formation rate of bacteria were significantly reduced on the NTAPPJ-treated titanium surfaces compared with the untreated samples, which was confirmed by fluorescent imaging. Regarding the comparison between gram-positive and gram-negative bacteria, both adhesion and the biofilm formation rate were significantly lower for gram-negative bacteria than gram-positive bacteria on samples treated for longer durations with the NTAPPJ. Transmission electron microscopy imaging showed a comparably more disruptive membrane structure of gram-negative bacteria than gram-positive bacteria on the NTAPPJ-treated surfaces. Our results indicated that the NTAPPJ treatment could be useful for preventing bacterial adhesion and biofilm formation on titanium dental implant surfaces, while the reductive potential on surfaces treated by the NTAPPJ could cause oxidation of bacteria, which could be more sensitive to gram-negative bacteria due to differences in the cell wall structure.