A 26-year-old man from Welywyn Garden City was stabbed to death following an argument on Sunday night.
Police were called to Guinness House, Little Hardings at approximately 6.35pm following a call from a member of the public and the man was pronounced dead at the scene.
A 52-year-old man, from Welwyn Garden City, has been arrested on suspicion of murder and is currently in police custody.
A police cordon has been put around the immediate area where the incident took place while police carry out inquiries.
This comes after the death of four-year-old Alexa-Marie, of Queen Bee Court in Hatfield, who was killed on Wednesday, March 12.
Chief Inspector for Welwyn Hatfield, Jo Walker said: "I am sure that the deaths currently being investigated in Welwyn Hatfield will be of concern to residents. These are highly unusual events for the borough and I would reassure people that we have a number of experienced detectives and specialist officers investigating them.
"We have one person charged in connection with the death of Alexa-Marie in Hatfield and a further person in custody for the death in Welwyn Garden City on Sunday. We do not believe the deaths are linked.
"My deepest sympathies go to the families of both victims. Specialist family liaison officers will be providing them with support at this extremely distressing time. If any residents do have concerns in relation to either of these deaths I would ask them to speak to their local Safer Neighbourhood Team by calling the non-emergency number, 101. If you have any information please report this to police via 101 or anonymously through the independent charity Crimestoppers on 0800 555111."
Any witnesses to, or anyone with information are asked to contact Detective Constable Donna Norris via the non-emergency number 101 citing crime reference B1/14/740. Alternatively call Crimestoppers anonymously on 0800 555 111.