The purpose of this thesis is to discover how the use of diegetic user interfaces (compared to non-diegetic user interfaces) affect immersion experienced in third-person video games. The main research question to be answered is: How does the use of diegetic user interfaces in third-person games affect the immersion experienced by players? To answer the main research question, the question was broken down into five different questions and online experiments were conducted. The experiments included six participants in which all of them played two different versions of the same computer game (one non-diegetic version and one diegetic version) as well as a survey (questionnaire) for each of the games including questions about the participants and numerical rating scales (on scales from 1 to 5) about their experiences with the game versions. In general, there did not seem to be a significant between the two versions of the game in terms of immersion. While the results suggest that the non-diegetic versions of the game causes players to become more frustrated, this could possibly just have been a coincidence due to the small sample size, and the difference was not “extreme” either (2.5 compared to about 1.833). Since the p-value generated was 0.32291 regarding frustration, which is several times more than 0.05, it suggests the statistical significance of the result is low. Enjoyability seemed to be about the same for the two versions of the game as well, and the variance was quite small compared to some other results (about 0.556). The variance regarding the visuals seems relatively small as well (0.25 to about 0.472) which tends to suggest that it might be unlikely that the non-diegetic user interface is heavily disliked. It might even be preferred. Either way, the results do not seem to be statistically significant.