The accident occurred when the right wingtip of Japan Airlines flight 68 (JAL68), a Boeing 787-9, collided with the vertical stabilizer of Delta Air Lines flight 1921 (DL1921), a Boeing 737-832, while taxiing to the gate after landing at the Seattle-Tacoma International Airport (SEA), Seattle, Washington. There were no injuries to passengers or crew on either airplane, and DL1921 sustained substantial damage to its empennage. The DL1921 flight crew reported that after pushback from gate A11 and taxiing via taxiway W, they were instructed to hold short of spot 99 and sequence for the south deice pad. DL1921 was stopped with the parking brake set abeam deice spot 1, waiting for marshalling and further clearance. According to the captain, the uncontrolled deice ramp was congested, slippery due to light snow and deicing fluid, and lacked defined hold points and marshalling guidance. The DL1921 flight crew stated that they were parked in position behind an Airbus A220 awaiting deice when they were struck from behind by a taxiing aircraft. The captain of DL1921 reported no prior indication that another aircraft was approaching from the rear and stated that he believed his aircraft was safely parked awaiting ground guidance. The first officer of DL1921 stated that he observed the approaching JAL68 aircraft’s wing moving uncomfortably close and alerted the captain, but the Delta aircraft could not move forward due to deice truck traffic and lack of a ground marshal. Moments later, the JAL68 aircraft’s right wingtip contacted the tail section of DL1921. According to statements from the JAL68 flight crew, they were taxiing toward gate S8 after landing on runway 16L. Due to congestion, the crew received multiple routing changes via ramp control, including taxi to spot 88 and subsequently back to spot 99 before re-entering the ramp toward S8. While maneuvering in the congested ramp near stands S15 and S16, the JAL68 captain (pilot flying) offset slightly left of centerline to maintain spacing with DL1921. The captain reported confirming with the first officer (pilot monitoring) whether the right wingtip was clear. The first officer responded that it appeared clear, but later recognized this was a misjudgment. The Japan Airlines crew estimated taxi speed at about 2 knots prior to contact. Both the captain and the observer crewmember reported they were aware of the proximity of the Delta aircraft but believed clearance would be maintained. The right wingtip of JAL68’s B-787 subsequently struck the tail of the parked DL1921. Following the collision, both crews initiated standard checklists. The Delta crew reported no injuries, secured the aircraft, and coordinated with Aircraft Rescue and Fire Fighting (ARFF) and airport operations for passenger deplaning via buses. The Japan Airlines crew stopped engines after receiving signals from Delta ground staff and reported the occurrence to ATC and company dispatch.