Bipartisan HSA Improvement Act of 2018 This bill amends the Internal Revenue Code to modify various requirements for health savings accounts (HSAs). The bill allows: individuals participating in an HSA to receive or be eligible to receive specified items and services at onsite employee clinics and retail health clinics, individuals who are covered by a spouse's health flexible spending arrangement to contribute to an HSA, high deductible health plans that are required for an HSA to include coverage with no deductible for care and prescription medications related to the treatment of medically complex chronic conditions, and HSAs to be used for certain sports and fitness expenses. The bill also: expands the definition of insurance that is permitted under an HSA to include insurance consisting of coverage for excepted benefits (Excepted benefits are not subject to certain requirements under the Patient Protection and Affordable Care Act.), modifies the definition of a dependent to include a child who has not attained the age of 27, and modifies the eligibility rules and other requirements for individuals who also participate in health flexible spending arrangements or health reimbursement arrangements.