A Study of JNJ-73763989 + Nucleos(t)Ide Analog in Participants Co-Infected With Hepatitis B and Hepatitis D Virus

Prior treatment

• Participants who have taken any therapies disallowed per protocol
• Participants who had or planned major surgery, (example, requiring general anesthesia) or who have received an organ transplant

Other illness

• Evidence of infection with hepatitis A, C, or E virus infection or evidence of human immunodeficiency, virus type 1 (HIV-1) or HIV-2 infection at screening
• Evidence of liver disease of non-HBV/HDV etiology
• Participants with a history of malignancy within 5 years before screening
• History of or current cardiac arrhythmia or history or clinical evidence of significant or unstable cardiac disease
• Participants with any current or previous illness for which, in the opinion of the investigator and/or sponsor, participation would not be in the best interest of the participant
• History of or current clinically significant skin disease or drug rash
• Participants with known allergies, hypersensitivity, or intolerance to JNJ-3989 or its excipients or excipients of the placebo content

Lab values

• History or evidence of clinical signs/symptoms of hepatic decompensation including but not limited to: portal hypertension, ascites, hepatic encephalopathy, esophageal varices or any laboratory abnormalities indicating a reduced liver function as defined in the protocol
• Significant laboratory abnormalities as defined in the protocol at screening

Other

• Signs of hepatocellular carcinoma (HCC)
• Abnormal sinus rhythm or ECG parameters at screening as defined in the protocol
• Contraindications to the use of entecavir (ETV), tenofovir disoproxil, or tenofovir alafenamide (TAF) per local prescribing information
• Female participants who are pregnant, or breast-feeding, or planning to become pregnant while enrolled in this study or within 90 days after the last dose of study intervention
• Male participants who plan to father a child while enrolled
• Vulnerable participants (example, incarcerated individuals, individuals under a legal protection measure)

• Medically stable based on physical examination, medical history, vital signs, electrocardiogram (ECG) at screening
• Chronic hepatitis B virus (HBV) and hepatitis D virus (HDV) co-infection with documentation at least 6 months prior to screening
• For Part 1: hepatitis D RNA (HDV RNA) greater than or equal to (\>=) 1000 international units per milliliter (IU/mL) at screening. For Part 2: must have HDV RNA values \>= 500 IU/mL, and must have hepatitis B surface antigen (HBsAg) values less than or equal to (\<=) 10000 IU/mL at screening or HDV RNA values at screening are \<= 100000 IU/mL
• Alanine aminotransferase (ALT) greater than upper limit normal (ULN) but less than 10 times (ULN)
• Body mass index (BMI) between 18.0 and 35.0 kilogram per meter square (kg/m\^2), extremes included
• Highly effective contraceptive measures in place for female participants of childbearing potential or male participants with female partners of childbearing potential
• Non-cirrhotic participants and participants with compensated cirrhosis (Child Pugh class A) at screening (Part 1) and participants must have absence of cirrhosis and platelet count of \>= 140000 per deciliter (dL) for enrollment into Part-2