Date: 202101
Local time of day: 0601-1200
Location: ZZZ.Airport, US
Flight conditions: Mixed
Weather elements / visibility: Cloudy
Light: Daylight
Aircraft: B737-700
Operator: Air Carrier
Operating under: Part 121
Flight plan: IFR
Mission: Passenger
Flight phase: Climb
Airspace: Class C ZZZ
Aircraft component: Exterior Pax/Crew Door
Component problem: Malfunctioning
Reporter organization: Air Carrier
Reporter function: Pilot Not Flying; Captain
Reporter qualification: Flight Crew Instrument; Flight Crew Multiengine; Flight Crew Air Transport Pilot (ATP)
Second reporter function: Pilot Flying; First Officer
Anomaly: Aircraft Equipment Problem Less Severe; Deviation / Discrepancy - Procedural Published Material / Policy; Flight Deck / Cabin / Aircraft Event Other / Unknown
Detector: Person Flight Attendant
When detected: In-flight
Result: Flight Crew Landed As Precaution; Flight Crew Overcame Equipment Problem
Contributing factors: Aircraft
Primary problem: Aircraft

Narrative: On climb out; after pressing the ATTEND CALL button as part of my flow; the A Flight Attendant immediately called the flight deck and complained of a loud squeal coming from the main door (L1) and that they could 'see daylight' coming from the door seal. I could hear the squeal in the background on the interphone. I looked on the overhead panel and verified that there was NO door light illuminated; I then went to verify pressurization and noticed that the cabin altitude was approaching 9;500 ft. I immediately instructed the First Officer to level off and then called ATC and reported a 'pressurization issue' and requested 10;000 ft.. We did not get a CABIN ALTITUDE WARNING at any point. I then requested to return to ZZZ and they gave us a further descent to 5;000 ft. and direct to the field. I made a PA and let the Passengers know that we were returning to ZZZ because of a pressurization issue and that we would be on the ground in ten minutes. I never requested priority handling because I didn't feel like it was necessary because we had already received an immediate descent clearance; but ATC declared one for us and had the trucks standing by on arrival. We had just enough time to run all of the checklists and get our landing data. The arrival and landing was uneventful. I called the Flight Attendants and asked how everyone was. The A and B Flight Attendants said they felt a 'little dizzy' but they said all Passengers seemed fine and there didn't seem to be any issues. Before opening the main door on arrival I inspected the seal and on the aft part of the door from top to bottom I could indeed see daylight and could run my hand in between the seal and door frame from top to bottom.


Second narrative: Loss of pressurization in the climb; due to forward entry door not sealed properly due to malfunction. Climbing through 10;000 ft. and beginning the Climb Checklist; the Flight Crew received a chime from the Cabin Crew that the forward entry door was squealing loud and air rushing through the front area enough causing the Flight Attendant's hair to be circulating with the flow of air. No indication of the event on the flight deck. Cabin pressurization reached 9;500 ft. The aircraft reached a peak altitude just over 12;000 ft. before descent.  Returned to ZZZ uneventful.  ATC themselves gave us priority handling for our return to ZZZ. Upon landing and at the gate; all Passengers were removed and no indications of any medical distress.


Synopsis: Flight Crew reported a diversion caused by an L1 door leak that prevented the cabin from pressurizing.