| Lou Nagelsen | |
| Position: | Catcher |
| Bats: | Right |
| Throws: | Right |
| Birth Date: | June 29, 1887 |
| Birth Place: | Piqua, Ohio, United States |
| Death Place: | Fort Wayne, Indiana, United States |
| Debutleague: | MLB |
| Debutdate: | September 10 |
| Debutyear: | 1912 |
| Debutteam: | Cleveland Naps |
| Finalleague: | MLB |
| Finaldate: | September 16 |
| Finalyear: | 1912 |
| Finalteam: | Cleveland Naps |
| Statleague: | MLB |
| Stat1label: | Batting average |
| Stat1value: | .000 |
| Stat2label: | Home runs |
| Stat2value: | 0 |
| Stat3label: | Runs batted in |
| Stat3value: | 6 |
| Teams: |
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Louis Marcellus Nagelsen (born Louis Marcellus Nagelsisen) (June 29, 1887 – October 21, 1965) was a Major League Baseball catcher who played for one season. He played for the Cleveland Naps for two games during the 1912 Cleveland Naps season. He attended the University of Notre Dame.