Ferritic stainless steel (FSS) is one of the high-temperature materials, used in many industries for sustainable applications such as power plants, automotive, offshore and chemical industries. Joining these materials is challenging due to the formation of an intermetallic and the grain growth with high-heat-input welding methods. Laser beam welding (LBW) that uses a low heat input was used successfully to join AISI 409 FSS tubes. In this work the welding speed and focal distance were varied as per a two-factor, three-level face-centred central composite design (FCCCD) to join AISI 409 FSS. A numerical model was developed to correlate the relationship between the ultimate tensile strength (UTS) and LBW process parameters. The validation of the developed model was carried out using the analysis of variance. Both welding speed and focal distance have a significant effect on determining the UTS. The optimised process parameters provided for a better UTS as reported in this paper.