A Study of Pembrolizumab (MK-3475) Versus Paclitaxel for Participants With Advanced Gastric/Gastroesophageal Junction Adenocarcinoma That Progressed After Therapy With Platinum and Fluoropyrimidine (MK-3475-061/KEYNOTE-061)

Inclusion Criteria:

* Histologically- or cytologically-confirmed diagnosis of gastric or gastroesophageal junction adenocarcinoma
* Confirmed metastatic or locally advanced, unresectable disease (by computed tomography \[CT\] scan or clinical evidence)
* Eastern Cooperative Oncology Group (ECOG) performance status of 0 or 1
* Progression on or after prior first-line therapy containing any platinum/fluoropyrimidine doublet
* Willing to provide tumor tissue for PD-L1 biomarker analysis (new or archived specimens with agreement of Sponsor). As of 20 March 2016, participants must be PD-L1 positive to be enrolled.
* Human epidermal growth factor receptor 2 (HER-2/neu) status known and participants with HER2/neu positive tumors show documentation of disease progression on treatment containing trastuzumab
* Female participants of childbearing potential should be willing to use 2 methods of birth control or be surgically sterile, or abstain from heterosexual activity for the course of the study through 120 days after the last dose of pembrolizumab or through 180 days after the last dose of paclitaxel.
* Male participants should agree to use an adequate method of contraception starting with the first dose of study therapy through 120 days after the last dose of pembrolizumab or through 180 days after the last dose of paclitaxel.
* Adequate organ function

Exclusion Criteria:

* Currently participating and receiving study therapy, or participated in a study of an investigational agent and received study therapy or used an investigation device within 4 weeks of the first dose of medication
* Squamous cell or undifferentiated gastric cancer
* Active autoimmune disease that has required systemic treatment in past 2 years (replacement therapy is not considered a form of systemic treatment
* Diagnosis of immunodeficiency or receiving systemic steroid therapy or any other form of immunosuppressive therapy within 7 days prior to the first dose of study medication
* Prior anti-cancer monoclonal antibody (mAb) within 4 weeks prior to study Day 1 or not recovered from AEs due to agents administered more than 4 weeks earlier
* Prior chemotherapy, targeted small molecule therapy, or radiation therapy within 2 weeks prior to study Day 1 or not recovered from adverse events due to a previously administered agent or surgery
* Known additional malignancy that is progressing or requires active treatment (with the exception of basal cell carcinoma of the skin, squamous cell carcinoma of the skin, or in situ cervical cancer that has undergone potentially curative therapy)
* Known active central nervous system (CNS) metastases and/or carcinomatous meningitis
* History of (noninfectious) pneumonitis that required steroids or current pneumonitis
* Active infection requiring systemic therapy
* Known psychiatric or substance abuse disorders that would interfere with cooperation with the requirements of the trial
* Pregnant or breastfeeding, or expecting to conceive or father children within the projected duration of the trial, starting with the screening visit through 120 days after the last dose of pembrolizumab or through 180 days after the last dose of paclitaxel.
* Prior immunotherapy including anti-PD-1, anti-PD-L1, or anti-PD-L2 agent, or previously participated in Merck pembrolizumab (MK-3475) clinical trial
* Known history of human immunodeficiency virus (HIV)
* Known active Hepatitis B or Hepatitis C
* Live vaccine within 30 days of planned start of study therapy
* Known allergy or hypersensitivity to paclitaxel or any components used in the paclitaxel preparation or other contraindication for taxane therapy