Modern aircraft systems use high-strength alloys, such as Ti-6Al-4V (Ti-6-4), for their components. The advantage of using Ti-6-4 is its high corrosion resistance and low weight to strength ratio when compared with other metals. The downside of using titanium is its high cost and lead time associated with procurement in addition to the material wastage that occurs with machining different cross-sectional geometries. Built-up titanium beams fabricated using friction stir welding (FSW) and readily available titanium plates may be a viable alternative that reduces these expenses. This study investigates the fatigue and static strengths of built-up titanium I-beams using FSW and 7-mm-thick Ti-6-4 plate material and compares the results with strengths of representative coupon specimens. Coupons in the transverse and longitudinal directions of the FSW were evaluated for both polished and as-welded FSW surfaces. The results of the investigation showed that the static flexural strength of the built-up I-beam reached 57 % of the yield strength of the Ti-6-4 plate material. The fatigue strengths of the two I-beams tested reached 87 and 63 % of comparable coupon-level specimens with as-welded and polished finishes, respectively. Failures in these beams initiated at discontinuities in the welded fillet region, which suggests that strength and fatigue performance of built-up titanium I-beams can be improved through more consistent FSW methods for Ti-6-4 plate materials.