Despite the availability of seasonal vaccines and antiviral medications, influenza virus continues to be a major health concern and pandemic threat due to the continually changing antigenic regions of the major surface glycoprotein, hemagglutinin (HA). One emerging strategy for the development of more efficacious seasonal and universal influenza vaccines is structure-guided design of nanoparticles that display conserved regions of HA, such as the stem. Using the H1 HA subtype to establish proof of concept, we found that an alpha-helical fragment (helix-A) from the conserved stem region can be displayed on nanoparticles. The stem region of HA on these nanoparticles is immunogenic and the nanoparticles are biochemically robust in that heat exposure did not destroy the particles and immunogenicity was retained. Furthermore, H1nanoparticles protected mice from lethal challenge with H1N1 influenza virus. Importantly, antibodies elicited by these nanoparticles demonstrated homosubtypic and heterosubtypic crossreactivity. The helix-A stem nanoparticle design represents a novel approach to display several hundred copies of non-trimeric conserved HA stem epitopes on vaccine nanoparticles. This design concept provides a new approach to universal influenza vaccine development strategies and opens up opportunities for the development of nanoparticles with broad coverage over many antigenically diverse influenza HA subtypes. SignificanceInfluenza virus is a public health issue that affects millions of people globally each year.Commercial influenza vaccines are based on the hemagglutinin (HA) surface glycoprotein, which can change antigenically every year, demanding the manufacture of newly matched vaccines annually. HA stem epitopes have a higher degree of conservation than HA molecules contained in conventional vaccine formulations and we demonstrate that we are able to design nanoparticles that display hundreds of HA stem fragments on nanoparticles. These designed nanoparticles are heat-stable, elicit antibodies to the HA stem, confer protection in mouse challenge models, and show cross-reactivity between HA subtypes. This technology provides promising opportunities to improve seasonal vaccines, develop pandemic preparedness vaccines, and facilitate the development of a universal influenza vaccine.