Background: As a result of the recent sustainability movement, consumers' attention has shifted away from the fast fashion industry and toward sustainable consciousness. Consumer awareness of the environmental impacts of the fast-fashion industry has recently increased, as evidenced by increased mentions of sustainability on social media. The increased awareness of the unsustainable conditions of the fast fashion industry and the exploitative environment of consumerism culture has resulted in an increase in awareness of the unsustainable conditions of consumerism culture. Despite the increased awareness, there hasn't been enough research focusing on the impact of sustainable activities on consumers from fast fashion companies. As a result, the purpose of this research paper is to address the main gap in the literature review, which is the effect of sustainable line extension in the fast fashion industry on consumer purchase intention.

Purpose: The following research question was formulated as a result of identified research gaps in this field concerning how sustainable line extension in the fast fashion industry affects consumers' purchase intention, as well as analyzing perception factors such as perceived brand attitude, perceived extension attitude, and perceived fit:

Method: This thesis aims to answer the stated research question under the positivist paradigm, by following the approach of deductive research. As a result, the data was gathered using a quantitative research method by creating a survey using a Google online questionnaire. This thesis's sample consists of Gen Y American consumers living on the West Coast.

Conclusion: The main result of this thesis is that sustainable line extensions presented by fast-fashion brands have a generally positive impact on fast-fashion consumer perception factors, resulting in positive purchase intentions, implying that fast-fashion and sustainability are not mutually exclusive concepts.