The Northern Formosan languages is a proposed grouping of Formosan languages that includes the Atayalic languages, the Western Plains languages (Papora, Hoanya, Babuza, and Taokas), and the Northwest Formosan languages (Pazeh and Saisiyat; Li places Western Plains with this grouping).

The Northern Formosan subgroup was first proposed by Paul Jen-kuei Li in 1985.^[1] Blust (1999) rejects the unity of the proposed Northern Formosan branch. A 2008 analysis of the Austronesian Basic Vocabulary Database, however, supports the unity of the Northern Formosan branch with a 97% confidence level (see Austronesian languages#Classification).

The following sound changes from Proto-Austronesian occurred in the Northern Formosan languages (Li 2008:215).^[2]

• *S₂, *H₁ > h
• *S₂, *H₁, *s > h (Atayalic languages and Saisiyat only)

Also, Pazeh, Saisiyat, and Thao are only Formosan languages that allow for SVO constructions, although this may be due to intensive contact with Taiwanese.^[3]

Also, the Atayal, Seediq, and Pazeh languages have devoiced final consonants that were present in the Proto-Austronesian (Blust 2009:616).

Li (2003, 2008) concludes the six western Plains languages split off from Proto-Northwestern Formosan. The classification is as follows.

The four coastal languages of Taokas, Babuza, Papora, and Hoanya share the following innovations (Li 2003).

1. Loss of *k
2. Loss of *-y
3. Merger of *s and *t in non-final position
4. Complete merger of *ŋ and *n

Thao shares the following innovations with the four coastal languages (Li 2003).

1. Merger of *s and *t
2. Merger of *ŋ and *n

Pazih has undergone the following two sound changes.

1. Merger of *j and *s as /z/
2. Merger of *C and *S₁ as /s/

Li (2003) does not consider Pazih to be very closely related to Saisiyat (Li 2003:946).

However, Shibata (2022) argues that the "Western Plains" languages in fact form a convergence area or linguistic area rather than a true subgroup.^[4]