Why is it die These?

These ends in -e, and most nouns ending in -e are feminine: in a German dictionary, 79% of them take die.

All nouns ending in -e
Sense 1
die

Meaning

[1] An unanswered assertion

[1] eine noch nicht bewiesene Behauptung

Examples

[1] Luther spread his theses in Wittenbergä

[1] Luther verbreitete seine Thesen in Wittenberg.

[1] "The mayor has to represent both theses at the same time: everything is more bad than ever - and everything is half as wild" "

[1] „Der Bürgermeister muss beide Thesen zugleich vertreten: Alles ist so schlimm wie noch nie – und alles ist halb so wild.“
More source examples (1)

These dictionary examples can include historical and less common uses.

[1] "Griebel took my theses without a hand"

[1] „Griebel nahm meine Thesen umstandslos an.“

Declension

1 Singular Plural
Nominativ die These die Thesen
Genitiv der These der Thesen
Dativ der These den Thesen
Akkusativ die These die Thesen

Pronunciation

Dictionary entries include material from Wiktionary. Wiktionary